PHPUnit 9 compatibility

[galbus]

When I attempt to composer require this package in my PHPUnit 9 project it fails with the following dependency/version error:

    - Root composer.json requires wp-graphql/wp-graphql-testcase ^2.1.3 -> satisfiable by wp-graphql/wp-graphql-testcase[v2.1.3].
    - wp-graphql/wp-graphql-testcase v2.1.3 requires phpunit/phpunit ^7.5 || ^8.5.0 -> found phpunit/phpunit[7.5.0, ..., 7.5.20, 8.5.0, ..., 8.5.26] but it conflicts with your root composer.json require (^9.5).

See Supported PHPUnit versions for PHPUnit's release timeline.

[justlevine]

Issue is that TraversableContains becomes an abstract in 9.x so we'd need a shim for TraversableContainsIdentical to maintain compat with 8.5 and below.
I'd offer, but phpunit is not my wheelhouse.

[kidunot89]

@galbus @justlevine This issue should be fixed with the release of v2.2.0

[justlevine]

@kidunot89 you sure ?

Its also causing WPGraphQL's core GH actions to fail (since wp-graphql-testcase was set to ~2.1, it gets updated to 2.3 by the docker entrypoint )

cc: @jasonbahl