The BinDiff core engine should have a test suite with reasonable coverage

BinDiff traditionally was only tested end-to-end running it on sets of BinExport files. While there is a large body of golden tests with expected results, any functional change to the core invalidates the existing test set. This makes implementation of new matching features or fixing long-standing issues unnecessarily hard.

Implement a test suite the has reasonable code coverage for the BinDiff core engine.

Internal bug: b/147221242