Use test fixtures in CheckSnapshot()

Using the preparer(s) that were used to run the test to verify the
integrity of the generated snapshot ensures that it will be verified in
the same environment as the snapshot was generated. This ensures that
as sdk tests are migrated to use fixtures that are optimized for each
test that they will benefit from those optimizations when checking the
snapshot.

Bug: 183184375
Test: m nothing
Change-Id: I62b383f9a1d9a77d1cabb101d9d1e4a976170fe3