Tests: add @group Windows to more tests

As part of PR 663, test runs on the Windows OS where introduced in CI/GH Actions.

While, generally speaking, all tests should run and pass on Windows too (and they do), for the purposes of CI, a specific test group named `Windows` was introduced to allow for selectively only running the tests which _may_ have a different outcome on Windows.
This group is used in the code coverage job to prevent the Windows builds taking forever.

While most PHPCS code is OS agnostic, there are a number of places in the code base where OS-based conditions are used.

This commit adds the `@group Windows` annotation to few more tests which hits these OS-based conditions to make sure those conditions are also hit in the code coverage builds.

Author: jrfnl

src/Standards/Generic/Tests/Debug/CSSLintUnitTest.php

@@ -16,6 +16,8 @@
  * Unit test class for the CSSLint sniff.
  *
  * @covers \PHP_CodeSniffer\Standards\Generic\Sniffs\Debug\CSSLintSniff
+ * @covers \PHP_CodeSniffer\Config::getExecutablePath
+ * @group  Windows
  */
 final class CSSLintUnitTest extends AbstractSniffUnitTest
 {

tests/Core/Fixer/GenerateDiffTest.php

@@ -21,6 +21,7 @@
  * test running the fixer itself, nor generating a diff based on a fixer run.
  *
  * @covers PHP_CodeSniffer\Fixer::generateDiff
+ * @group  Windows
  */
 final class GenerateDiffTest extends TestCase
 {