Refactor TestThrottle to avoid incorrect Fatal usage.
Refactor TestThrottle to avoid incorrect Fatal usage.

Fatal is not safe for concurrent usage and may only be called by the
testing goroutine, not new goroutines spawned by the test.

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