MetadataManagerTest must reset `orphanCleanupDelaySecs` to the default value on teardown

The metadata manager test fixture setup is overriding the default value of orphanCleanupDelaySecs causing all subsequent unit tests to see the newly set value.

Purpose of this ticket is to reset the default value during teardown of such fixture. Something similar to what range deleter service tests already do, but at fixture level rather than at single unit test cases level.