Add test of intersecting manifolds again, now hopefully running in
parallel.
Merge remote-tracking branch 'origin/meg/test-intersecting-manifolds' into aslakbergersen/topic-change-unittest-to-pytest

Conflicts:
	test/unit/fem/python/manifolds.py
Bugfix in test_manifolds rewritten to pytest in merge.
Create fixtures 'rootdir' and 'datadir', move downloaded mesh files to
directory test/data/ which is referenced from tests via
'datadir'. This is a precursor to moving tests a bit.

Also changed cpp test runners a bit, something broke. These will be
replaced later though.
Move unit/foo/python to unit/python/foo.