tests: Fix filter test cases (broken due to 937ec9be)
tests: Add test case showing bad behaviour with nested #includes

Two issues:

include1.h is filtered out, but the #include statement from
includeinclude1.h is not preserved into the .bi.

does-not-exist.h is not found, but the #include statement from
includenotfound.h is not preserved into the .bi.
Fix handling of #includes from included files (1ae63462 regression)

fbfrog only preserved #include statements from the toplevel file(s),
but not #include statements from included files. This is wrong though;
it doesn't matter whether the context is toplevel or not, the only thing
that matters is whether the #include statement will be filtered out or not.