On (not as rare as we'd like) occassion, we observe clocks moving backwards in our CI environment.

There are many problems with this, but in particular, it can cause go test to emit negative test runtimes (see cockroachdb/cockroach#10937, which contains:

--- FAIL: TestServerQueryStarvation (-0.13s)
	server_test.go:262: error storing data for series 22, source 4: storage/store.go:2257: rejecting command with timestamp in the future: 1479800017448958100 (281.69074ms ahead)
FAIL

).

This is due to the use of a non-monotonic clock in:

• https://github.com/golang/go/blob/26827bc/src/testing/testing.go#L601
• https://github.com/golang/go/blob/26827bc/src/testing/testing.go#L635

and perhaps others.

This issue is related to #12914.