sanitycheck: tests: Fill results for skipped tests

Sanitycheck discards test instances if a set of given coditions
is not fulfilled. This leads to empty test results for these
instances. This can introduce ambiguity with results that are
missing due to some bugs in the framework. This commit fills
the results for skipped tests with 'skipped' states and provides
the reason for filtering them out in the msg field. The commit
also fixes the way sanitycheck counts and reports tests/test cases

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>