Add support for PHPUnit 10

[owenvoke]

There is an upcoming major version bump for PHPUnit. This seems to contain some breaking changes that affect Pest.

• Docs for v10
• Branch for v10
• Changelog for v10
• Diff for v10

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Current issues

• Custom TestSuiteLoader deprecation
  I think that this is now indirectly causing issues as it now seems to expect any selected file in the testsuite directory to contain a PHP class. So if the testsuite directory is set to tests/ and suffix=".php" (as per our current suite), it tries loading all files that are pascal case as a class. This means that tests/Autoload.php is loaded as a class, as are all the datasets, etc.

  This basically means that all Pest-syntax tests will fail, as they don't contain a class in the files. 😬

[nunomaduro]

Closing in favour of #263