AssertStringContains: improve handling of custom error message

Some of the assertions in the `AssertStringContains` trait generate a specific error message.
However, if a custom `$message` parameter was passed, that message would not be displayed and only the custom `$message` would be shown when the assertion failed.

As the PHPUnit native error message from the _underlying_ assertion used - _"true is not false"_ - would be obscuring the real issue, this commit adjusts the code to always display the assertion specific error message as well.

Includes tests safeguarding the fix.

src/Polyfills/AssertStringContains.php

@@ -69,11 +69,12 @@ public static function assertStringContainsStringIgnoringCase( $needle, $haystac
 	 */
 	public static function assertStringNotContainsString( $needle, $haystack, $message = '' ) {
 		if ( $needle === '' ) {
-			if ( $message === '' ) {
-				$message = "Failed asserting that '{$haystack}' does not contain \"{$needle}\".";
+			$msg = "Failed asserting that '{$haystack}' does not contain \"\".";
+			if ( $message !== '' ) {
+				$msg = $message . \PHP_EOL . $msg;
 			}
 
-			static::fail( $message );
+			static::fail( $msg );
 		}
 
 		static::assertNotContains( $needle, $haystack, $message );
@@ -90,11 +91,12 @@ public static function assertStringNotContainsString( $needle, $haystack, $messa
 	 */
 	public static function assertStringNotContainsStringIgnoringCase( $needle, $haystack, $message = '' ) {
 		if ( $needle === '' ) {
-			if ( $message === '' ) {
-				$message = "Failed asserting that '{$haystack}' does not contain \"{$needle}\".";
+			$msg = "Failed asserting that '{$haystack}' does not contain \"\".";
+			if ( $message !== '' ) {
+				$msg = $message . \PHP_EOL . $msg;
 			}
 
-			static::fail( $message );
+			static::fail( $msg );
 		}
 
 		static::assertNotContains( $needle, $haystack, $message, true );

tests/Polyfills/AssertStringContainsTest.php

@@ -5,6 +5,7 @@
 use PHPUnit\Framework\TestCase;
 use Yoast\PHPUnitPolyfills\Polyfills\AssertStringContains;
 use Yoast\PHPUnitPolyfills\Polyfills\ExpectException;
+use Yoast\PHPUnitPolyfills\Polyfills\ExpectExceptionMessageMatches;
 
 /**
  * Availability test for the functions polyfilled by the AssertStringContains trait.
@@ -15,6 +16,7 @@ final class AssertStringContainsTest extends TestCase {
 
 	use AssertStringContains;
 	use ExpectException; // Needed for PHPUnit < 5.2.0 support.
+	use ExpectExceptionMessageMatches;
 
 	/**
 	 * Verify availability of the assertStringContainsString() method.
@@ -145,6 +147,36 @@ public static function dataHaystacks() {
 		];
 	}
 
+	/**
+	 * Verify that the assertStringNotContainsString() method fails a test with a custom failure message,
+	 * when the custom $message parameter has been passed.
+	 *
+	 * @return void
+	 */
+	public function testAssertStringNotContainsStringFailsWithCustomMessage() {
+		$pattern = '`^This assertion failed for reason XYZ\s+Failed asserting that \'.+?\' does not contain ""\.`s';
+
+		$this->expectException( $this->getAssertionFailedExceptionName() );
+		$this->expectExceptionMessageMatches( $pattern );
+
+		$this->assertStringNotContainsString( '', 'something', 'This assertion failed for reason XYZ' );
+	}
+
+	/**
+	 * Verify that the assertStringNotContainsStringIgnoringCase() method fails a test with a custom failure message,
+	 * when the custom $message parameter has been passed.
+	 *
+	 * @return void
+	 */
+	public function testssertStringNotContainsStringIgnoringCaseFailsWithCustomMessage() {
+		$pattern = '`^This assertion failed for reason XYZ\s+Failed asserting that \'.+?\' does not contain ""\.`s';
+
+		$this->expectException( $this->getAssertionFailedExceptionName() );
+		$this->expectExceptionMessageMatches( $pattern );
+
+		$this->assertStringNotContainsStringIgnoringCase( '', 'something', 'This assertion failed for reason XYZ' );
+	}
+
 	/**
 	 * Helper function: retrieve the name of the "assertion failed" exception to expect (PHPUnit cross-version).
 	 *