Add more comprehensive tests for abi diffing.

1) We do not involve the build system to create test reference dumps. Symbols
in tests, are faked through version scripts to avoid checking in prebuilt .so
test files.

2) Also add a script to create reference abi dumps.

3) Fix a comment in ast_processing.cpp

Test: Run tests/test.py, all tests run and pass on generic arm64, mips64
and x86_64 builds.

Change-Id: Id3a420c1ae093a05a79da4c29e85071f28d714d1