Add unit test for parsing build files in bp2build

This involves some minor changes to testing infrastructure.

This is a rollforward of aosp/2628496 with a couple of minor changes:

- In ParseBuildFiles, filter out all build files that are kept due
  to ShouldKeepExistingBuildFileForDir

- Add some minor test infrastructure for StubbedBuildDefinitions, with a
  couple of proof of concept tests used to demonstrate its usage. This
  pattern will become immensely more common as we implement allowlist v2
  (as we will need to update all tests which today simulate build
  definitions that have missing deps)

Bug: 285631638
Fixes: 286545783
Test: bp2build.sh
Test: m nothing
Change-Id: I7c3a03b02098e39dd8e51d327482b440f294478f