Refactor test to avoid data race.
Refactor test to avoid data race.

This refactors the udb TestUpgrades function to avoid a data race
(potentially a Go issue, see golang/go#20339) by replacing the
concurrent t.Run and sync.WaitGroup usage with a subtest composed of
other parallel subtests.  This allows cleanup to be run outside of the
outer subtest.

decred/dcrwallet,jrick/dcrwallet,jrick/dcrwallet,decred/dcrwallet,jrick/btcwallet,jrick/btcwallet