Ruleset Tests: Work around WPCS trim() bug

PHP 8.1 increased the strictness of types used with native functions, and this meant WPCS 2.3.0 had a bug: WordPress/WordPress-Coding-Standards#2035

For the VIPCS ruleset tests with PHP 8.1 or above it would skip over a bunch of lines, and mark other lines as expecting errors or warnings incorrectly.

Running:

```
vendor/squizlabs/php_codesniffer/bin/phpcs --standard=WordPressVIPMinimum --severity=1 WordPressVIPMinimum/ruleset-test.inc -s
```

...shows all the expected violations, but also this for line 1:

> An error occurred during processing; checking has been aborted. The error message was: trim(): Passing null to parameter #1 ($string) of type string is deprecated in .../vipcs/vendor/wp-coding-standards/wpcs/WordPress/Sniff.php on line 1144 (Internal.Exception)

Line 1144 relate to the retrieval of the `minimum_supported_wp_version` config value. Since it appeared to be coming through as null, the RulesetTest.php now includes setting this as a command-line argument when PHPCS is called to run the ruleset test.

When WPCS 3.0 is the minimum supported, these changes can be reverted.

tests/RulesetTest.php

@@ -146,7 +146,7 @@ private function collect_phpcs_result() {
 		}
 
 		$shell = sprintf(
-			'%1$s%2$s --severity=1 --standard=%3$s --report=json ./%3$s/ruleset-test.inc',
+			'%1$s%2$s --severity=1 --standard=%3$s --report=json --runtime-set minimum_supported_wp_version 0 ./%3$s/ruleset-test.inc',
 			$php, // Current PHP executable if available.
 			$this->phpcs_bin,
 			$this->ruleset