libsnapshot: Fix MergeCannotRemoveCow

The test is broken because now ProcessUpdateState
checks UPDATED flag as well, and partition "test-snapshot"
does not exist in super partition metadata.

Rewrite the test to mimic the update flow and use
as many public APIs as possible.

Test: libsnapshot_test
Bug: 143551390

Change-Id: I105bad8bb589616b304c511c64be902d4297bda7