sanitycheck: Add filtered test cases to the JUnit full report

If a test instance from .yaml file was skipped due to being on
a filtered list then there are no entries about it in the
sanitycheck_report.xml for a given platform. This commit fills
instance.results with skipped test cases also for skipped instances so
those test cases are visible when the full report is generated. Tested
for ./tests/kernel/mem_protect/userspace on nrf52dk_nrf52832 where
userspace.gap_filling is filtered out.

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