fix: tests defer database cleanup without invoking

[iand]

A recent refactoring wrapped the deferred database cleanups in an error assertion but defer evaluates arguments at the call site so this was invoking the cleanup function immediately.

[alanshaw]

I'm getting a bunch of errors running this:

VISOR_TEST_DB="postgres://alan@localhost/visor_test?sslmode=disable" go test ./storage/...

--- FAIL: TestMarkTipSetComplete (0.01s)
    sql_test.go:1057: 
                Error Trace:    sql_test.go:155
                                                        sql_test.go:1057
                Error:          Received unexpected error:
                                ERROR #42P01 relation "visor_processing_tipsets" does not exist
                Test:           TestMarkTipSetComplete
                Messages:       truncating visor_processing_tipsets