libsnapshot_fuzzer: Add tests

Add tests for the fuzzer itself. The test ensures that the
initial corpus is a good run on SnapshotManager (in the sense
that not only it doesn't crash, but also it executes as expected).

Also changed fuzz_utils so that impl functions return the
invocation result too, so that the test can check its values.

Test: run it
Bug: 154633114

Change-Id: I31ab7830f8c20a0575aaadabd038632d10f33962