.gitattributes: ignore test files, don't ignore test framework

As of PHPCS 4.0, the tests will no longer ship in a packaged version of the repo 🎉

However, while this removes a lot of friction, it can also cause some, as quite a few external standards use the PHPCS native test framework to run their tests. To continue to do so, those external standards would now have to use `--prefer-source`, which would make their builds a lot slower and will cause extra support questions from new contributors to those standards.

This commit proposes a solution to this issue by making a few small changes to the `.gitattributes` file.
With these updates directives in place, the PHPCS native test _framework_ will still be included in the packaged up versions, but the test _files_ will not be.

In practice, this means that the following files should still be included in a packaged up version:
* `/tests/AllTests.php`
* `/tests/bootstrap.php`
* `/tests/FileList.php`
* `/tests/TestSuite.php`
* `/tests/Core/AbstractMethodUnitTest.php`
* `/tests/Core/AllTests.php`
* `/tests/Standards/AbstractSniffTest.php`
* `/tests/Standards/AllSniffs.php`

... but that all other test files will be excluded.

This removes the need for external standards to use `--prefer-source`.

.gitattributes

@@ -5,16 +5,18 @@
 # https://www.reddit.com/r/PHP/comments/2jzp6k/i_dont_need_your_tests_in_my_production/
 # https://blog.madewithlove.be/post/gitattributes/
 #
-.github/                export-ignore
-scripts/                export-ignore
-src/Standards/**/Tests/ export-ignore
-tests/                  export-ignore
-.cspell.json            export-ignore
-.gitattributes          export-ignore
-.gitignore              export-ignore
-phpcs.xml.dist          export-ignore
-phpstan.neon.dist       export-ignore
-phpunit.xml.dist        export-ignore
+.github/                            export-ignore
+scripts/                            export-ignore
+src/Standards/**/Tests/             export-ignore
+tests/Core/**/                      export-ignore
+.cspell.json                        export-ignore
+.gitattributes                      export-ignore
+.gitignore                          export-ignore
+phpcs.xml.dist                      export-ignore
+phpstan.neon.dist                   export-ignore
+phpunit.xml.dist                    export-ignore
+tests/Core/ErrorSuppressionTest.php export-ignore
+tests/Core/IsCamelCapsTest.php      export-ignore
 
 #
 # Declare files that should always have CRLF line endings on checkout.