Ensure that `wpdb_exposed_methods_for_testing` is available to all tests.

It should be defined in a common library file so that all tests have access to
it, even when run in isolation.

See [30345], #21212.

git-svn-id: https://develop.svn.wordpress.org/trunk@30528 602fd350-edb4-49c9-b593-d223f7449a82