Repair mssql dep tests; have __only_on__ imply __backend__

Repair mssql dep tests; have __only_on__ imply __backend__

CI missed a few SQL Server tests because we run mssql-backendonly
in the gerrit job.   As there was a test that was "only on"
mssql but didn't have backendonly, it never got run and then
fails in master where we run mssql fully.

Any suite that has an __only_on__ is inherently specific to
a backend, so if present this should imply __backend__ so that
it definitely runs when we have that backend present.

This in turn meant we had to fix a few sqlite_file tests that
weren't cleaning up or sharing well as they suddenly became
backend tests under sqlite_file.  Added a sqlite_file cleanup
to test class cleanup for now.

Change-Id: I9de1ceabd6596547a65c59059a55b7e5156103fd