Site Health: Ensure that the tests returned by `WP_Site_Health::get_tests()`...

Site Health: Ensure that the tests returned by `WP_Site_Health::get_tests()` always have the required array keys: `direct` and `async`.

This avoids PHP notices if these keys were accidentally removed using the `site_status_tests` filter.

Props khag7, Ov3rfly, desrosj, Clorith.
Merges [48808] to the 5.5 branch.
Fixes #50858.
Built from https://develop.svn.wordpress.org/branches/5.5@48810


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48572 1a063a9b-81f0-0310-95a4-ce76da25c4cd