cmd/go/internal/test: scale the WaitDelay for test subprocesses
Prior to CL 456116 we had an arbitrary 5-second delay after a test
times out before we kill the test. In CL 456116, I reused that
arbitrary 5-second delay as the WaitDelay as well, but on slower
builders it does not seem to be generous enough.
Instead of hard-coding the delay, for tests with a finite timout we
now use a hard-coded fraction of the overall timeout. That will
probably give delays that are longer than strictly necessary for very
long timeouts, but if the user is willing to wait for a very long
timeout they can probably wait a little longer for I/O too.