This updates some minor issues that would cause warnings or make things appear to not work, even though they would work fine when actually running a backup.
This PR adds a flag to the test method so we can choose to test for read-only access (needed for restore) or read-write (needed for backups).
Additionally, this PR fixes a logic bug in that the BackendSourceProvider would not actually test the connection, but just check the placeholder element returned (which is always returned). With this PR the backend's ListAsync command is tested fully.
Also renamed the methods on ISourceProvider to have the `...Async` prefix.
This replaces #6939