iotests: Add casenotrun report to bash tests

iotests: Add casenotrun report to bash tests

The new function _casenotrun() is to be invoked if a test case cannot
be run for some reason. The user will be notified by a message passed
to the function. It is the caller's responsibility to make skipped a
particular test.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>