tapdb+mssmt: update mssmt unit tests to run against all registered db backends
Before this PR, we had the logic to execute all registered dbs, but didn't actually register them all. With this PR, we now call RegisterTreeStore.
By default the mssmt unit tests will run sqlite. You an run postgres with this build tag: -tags=test_db_postgres.
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@roasbeef, remember to re-request review from reviewers when ready
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