coverage reports 0% coverage

[KrisThielemans]

1149 re-introduced running of coverage, and pushing to coveralls.io. However, there is one thing that doesn't work...

coverage computed correctly for each subdir, e.g. output of the pytest runs

: ----------- coverage: platform linux, python 3.6.9-final-0 -----------
6: Name                           Stmts   Miss  Cover
6: --------------------------------------------------
6: src/Registration/pReg/Reg.py     838    212    75%
6: --------------------------------------------------
6: TOTAL                            838    212    75%
....
- ----------- coverage: platform linux, python 3.6.9-final-0 -----------
2: Name                                     Stmts   Miss  Cover
2: ------------------------------------------------------------
2: src/xGadgetron/pGadgetron/Gadgetron.py    1077    779    28%
2: ------------------------------------------------------------
2: TOTAL                                     1077    779    28%

There are no absolute paths anymore in the .coverage-* files, so this should all be fine.

However, the output of coverage combine then gets rid of the coverage itself, e.g.

Name                                     Stmts   Miss  Cover
------------------------------------------------------------
src/Registration/pReg/Reg.py               838    838     0%
src/xGadgetron/pGadgetron/Gadgetron.py    1077   1077     0%
------------------------------------------------------------
TOTAL                                     1915   1915     0%

Originally posted by @KrisThielemans in https://github.com/SyneRBI/SIRF/issues/1149#issuecomment-1329050916