Fix pinned WordPress version

[alecgeatches]

This is a PR to undo a change made to temporarily fix tests. At the time of writing, there is a file naming mismatch between WordPress' latest release and the latest version of the testing library pulled from tests/phpunit. This causes tests to fail, and can be fixed by specifying a release WordPress version.

Once WordPress 6.1 is released this should no longer be an issue. When that happens, ensure tests pass in this PR and merge.

See this discussion in this PR for details: https://github.com/Automattic/vip-decoupled-bundle/pull/57#issuecomment-1202067748.

[alecgeatches]

@chriszarate This has been fixed in wp-env and tests are now fixed. I submitted a PR for this fix in https://github.com/WordPress/gutenberg/pull/43020, but it ended up being fixed in a concurrent issue https://github.com/WordPress/gutenberg/pull/42826 in a similar but more robust and better tested way. The original issue didn't yet exist when I first looked into solving this problem and unfortunately I was not aware of the overlap. Next time I'll run through issues a second time when I start actual development, especially when there's a fresh bug like that.

The WordPress version is now properly fixed to the phpunit version and we're good to unpin WordPress here.

[chriszarate]

Next time I'll run through issues a second time when I start actual development, especially when there's a fresh bug like that.

Can't say I'd recommend that! 😂