Add option for "sparse" backend tests and apply to memusage

Add option for "sparse" backend tests and apply to memusage

The memusage tests are extremely time and memory intensive,
and when CI runs against MySQL or Postgresql there are many
database/driver combinations for which the "backend" tests
repeatedly run; as these tests are more oriented towards
basic dialect interaction, add a new "sparse" backend option
that will run the tests only once per base dialect.

Change-Id: I312aa0332d7ec1ff4e2faa15f6b189d6f0f68393