libsnapshot_fuzzer: add initial corpus

Transform some of the tests in vts_libsnapshot_test
to corpus and use them as initial corpus to
libsnapshot_fuzzer.

The corpus alone gives us 50% of line coverage in snapshot.cpp.

Test: run it
Bug: 154633114
Change-Id: I8f3bf1d76ef64d710224e24c913990692481b65e