test: add test case for verity deferred removal without sharing

test: add test case for verity deferred removal without sharing

I recently found out (the hard way) that on an older version
there was a bug when the verity sharing is disabled: the
deferred close flag was not set correctly, so verity devices
were leaked.

This is not an issue in main currently, but add a test case
to cover it just in case, to avoid future regressions.