tests/frontend/overlaps.py: Added regression test for cross project overlaps

This is a regression test against issue #926 (closed), which occurred in 1.2 but is already fixed in master.

The fix in master was an accidental consequence of implementing configurable warnings, since we trust the self.warn() API to safely decide on whether a warning is fatal or not, we no longer make the mistake of asking the overlapping element's project fail-on-overlap policy but automatically consult the correct self project (the project associated with the element being assembled).