lsifstore: do not use dbconn.Global in tests
Created by: mrnugget
I came across this while putting together #26641 and realised that it's
pretty easy to switch to dbtest
here.
Runtime difference: new version is ~300ms slower.
But I think it's worth it for now, since migrating with everything to
dbtest.NewDB
brings us closer to the next step: optimizing
dbtest.NewDB
.