Add a test for bug 198265278.

This test uses dependency injection to cause a failure in
QuerySnapshotStatus. We expect ProcessUpdateState to return MergeFailed.
The test also checks that if the merge is attempted again, it can
succeed.

Some duplicated code has also been factored out into the test harness.

Bug: 198265278
Test: vts_libsnapshot_test gtest
Change-Id: I6ccb434afa0e5ebf6781b2cec5277e3b7c210b77