Add more Soong unit test coverage for gensrcs

When gensrcs's srcs prop is set with filegroup from external package and especially when gensrcs generates cpp headers, it's not trivial to know what the output paths and their correponding include paths look like.

In mixed build, there are a few bugs in gensrcs's include paths for cpp headers (b/248555356 and b/248554581) that we'll eventually need to fix. These tests serve as an documentation of the existing behavior when gensrcs generate cpp headers.

Test: go test
Bug: 248555356
Change-Id: I10168dd4229be8f110a31955d214ef792c8050de