Give the terminate tests a margin, without letting them stop failing

[SimonCropp]

Both asserted WaitForExit(1000) after terminating a process. One second is a bet
on the runner being idle, and on a loaded two core Windows runner it is not: the
pair went red on main, and a re-run with no code change went green.

Widening alone would have been wrong for the non-windowed case. That
FakeDiffTool sleeps five seconds and then exits by itself, so any wait past five
seconds passes whether the process was terminated or merely ran out - the same
trap LaunchAndKill was in. So that one also asserts the exit code:
TryTerminateProcess passes -1 to TerminateProcess, and running out returns 0.

The windowed case needs no such guard, since a windowed FakeDiffTool runs until
something closes it. Waiting longer there cannot turn a failure into a pass.

Checked by making TryTerminateProcess skip the terminate: both tests fail, one
on the exit code and one on the wait.