test: Let pytest indicate skipped C unit tests

test: Let pytest indicate skipped C unit tests

We invoke the ut command in test_ut.py. Currently we only check for
failures. Instead we should also indicate if sub-tests were skipped.

With this change we will get output like the following for skipped tests:

test/py/tests/test_ut.py ..sssss......ss..............s.sssss.s.s...
================================ short test summary info ================================
SKIPPED [1] test/py/tests/test_ut.py:597: Test addrmap addrmap_test_basic has 1 skipped sub-test(s).
SKIPPED [1] test/py/tests/test_ut.py:597: Test bdinfo bdinfo_test_eth has 4 skipped sub-test(s).

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>