Adds a RandomContext for generating random strings in steps.

```
Given I am viewing an "Article" with the title "<title>"
Then I should see the header "<title>"
```

When the `RandomContext` is included, then `<title>` will be replaced
with a random string during the scenario. It will have a consistent
value throughout a given scenario, but a new value in subsequent
scenarios.

- Fixes #293

behat.yml.dist

@@ -115,6 +115,7 @@ drupal8:
         - Drupal\DrupalExtension\Context\MarkupContext
         - Drupal\DrupalExtension\Context\MessageContext
         - Drupal\DrupalExtension\Context\MailContext
+        - Drupal\DrupalExtension\Context\RandomContext
       filters:
         tags: "@d8&&~@d8wip"
   extensions:

doc/contexts.rst

@@ -31,6 +31,11 @@ following contexts:
   Step-definitions that are specific to Drupal messages that get displayed
   (notice, warning, and error).
 
+*RandomContext*
+  Contains transforms that allow for use of placeholders such as `<title>` that will be replaced with a random string
+  when the scenario is run: `Given I am viewing an "Article" with the title "<title>"`
+
+
 *DrushContext*
   Allows steps to directly call drush commands.
 

features/random.feature

@@ -0,0 +1,20 @@
+@api @d8 @random
+Feature: RandomContext functionality
+  In order to prove the RandomContext is functional at transforming variables
+  As a developer
+  I need to use random variables in scenarios
+
+  # This will fail on the second scenario if random transforms are not functional.
+  Scenario: Create a first user
+    Given I am at "/user/register"
+    And I fill in "Email address" with "<user>@example.com"
+    And I fill in "Username" with "<user>"
+    When I press "Create new account"
+    Then an email has been sent to "<user>@example.com" with the subject "Account details for <user>"
+
+  Scenario: Create the second user
+    Given I am at "/user/register"
+    And I fill in "Email address" with "<user>@example.com"
+    And I fill in "Username" with "<user>"
+    When I press "Create new account"
+    Then an email has been sent to "<user>@example.com" with the subject "Account details for <user>"

spec/Drupal/DrupalExtension/Context/RandomContextSpec.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace spec\Drupal\DrupalExtension\Context;
+
+use Drupal\DrupalExtension\Context\RandomContext;
+use PhpSpec\ObjectBehavior;
+use Prophecy\Argument;
+
+class RandomContextSpec extends ObjectBehavior
+{
+    function it_is_initializable()
+    {
+        $this->shouldHaveType(RandomContext::class);
+    }
+}

src/Drupal/DrupalExtension/Context/RandomContext.php

@@ -0,0 +1,88 @@
+<?php
+
+namespace Drupal\DrupalExtension\Context;
+
+use Behat\Behat\Hook\Scope\AfterScenarioScope;
+use Behat\Behat\Hook\Scope\BeforeScenarioScope;
+
+/**
+ * Class RandomContext
+ * @package Drupal\DrupalExtension\Context
+ *
+ * Transform tokens into random variables.
+ */
+class RandomContext extends RawDrupalContext
+{
+    /**
+     * Tracks variable names for consistent replacement during a given scenario.
+     *
+     * @var array
+     */
+    protected $values = [];
+
+    /**
+     * The regex to use for variable replacement.
+     */
+    const VARIABLE_REGEX = '#(\<.*?\>)#';
+
+    /**
+     * Transform random variables.
+     *
+     * @Transform #([^<]*\<.*\>[^>]*)#
+     */
+    public function transformVariables($message)
+    {
+        $patterns = [];
+        $replacements = [];
+
+        preg_match_all(static::VARIABLE_REGEX, $message, $matches);
+        foreach ($matches[0] as $variable) {
+            $replacements[] = $this->values[$variable];
+            $patterns[] = '#' . $variable . '#';
+        }
+        $message = preg_replace($patterns, $replacements, $message);
+
+        return $message;
+    }
+
+    /**
+     * Set values for each random variable found in the current scenario.
+     *
+     * @BeforeScenario
+     */
+    public function beforeScenarioSetVariables(BeforeScenarioScope $scope)
+    {
+        $steps = [];
+        if ($scope->getFeature()->hasBackground()) {
+            $steps = $scope->getFeature()->getBackground()->getSteps();
+        }
+        $steps = array_merge($steps, $scope->getScenario()->getSteps());
+        foreach ($steps as $step) {
+            preg_match_all(static::VARIABLE_REGEX, $step->getText(), $matches);
+            $variables_found = $matches[0];
+            // Find variables in are TableNodes or PyStringNodes.
+            $step_argument = $step->getArguments();
+            if (!empty($step_argument) && $step_argument[0] instanceof TableNode) {
+                preg_match_all(static::VARIABLE_REGEX, $step_argument[0]->getTableAsString(), $matches);
+                $variables_found = array_filter(array_merge($variables_found, $matches[0]));
+            }
+            foreach ($variables_found as $variable_name) {
+                if (!isset($this->values[$variable_name])) {
+                    $value = $this->getDriver()->getRandom()->name(10);
+                    // Value forced to lowercase to ensure it is machine-readable.
+                    $this->values[$variable_name] = strtolower($value);
+                }
+            }
+        }
+    }
+
+    /**
+     * Reset variables after the scenario.
+     *
+     * @AfterScenario
+     */
+    public function afterScenarioResetVariables(AfterScenarioScope $scope)
+    {
+        $this->values = [];
+    }
+}