Unit tests: Fix PHPUnit deprecation

When running the unit tests with PHP 8.1 or above, PHPUnit would give a deprecation notice:

> PHP Deprecated:  strlen(): Passing null to parameter #1 ($string) of type string is deprecated in .../vipcs/vendor/phpunit/phpunit/src/Util/Getopt.php on line 159

The tests still ran successfully though.

This notice originated from the use of `--filter WordPressVIPMinimum`, and was fixed by adding an equals sign: `--filter=WordPressVIPMinimum`.

Under both versions, the same number of tests and test files and unique error codes were created.

bin/unit-tests

@@ -17,7 +17,7 @@
 #
 
 if [[ $(php -r 'echo PHP_VERSION_ID;') -ge 80100 ]]; then
-	"$(pwd)/vendor/bin/phpunit" --filter WordPressVIPMinimum "$(pwd)/vendor/squizlabs/php_codesniffer/tests/AllTests.php" --no-coverage --no-configuration --bootstrap=./tests/bootstrap.php --dont-report-useless-tests $@
+	"$(pwd)/vendor/bin/phpunit" --filter=WordPressVIPMinimum "$(pwd)/vendor/squizlabs/php_codesniffer/tests/AllTests.php" --no-coverage --no-configuration --bootstrap=./tests/bootstrap.php --dont-report-useless-tests $@
 else
-	"$(pwd)/vendor/bin/phpunit" --filter WordPressVIPMinimum "$(pwd)/vendor/squizlabs/php_codesniffer/tests/AllTests.php" --no-coverage $@
+	"$(pwd)/vendor/bin/phpunit" --filter=WordPressVIPMinimum "$(pwd)/vendor/squizlabs/php_codesniffer/tests/AllTests.php" --no-coverage $@
 fi