internal/testenv: adjust timeout calculations in CommandContext
I noticed some test failures in the build dashboard after CL 445597
that made me realize the grace period should be based on the test
timeout, not the Context timeout: if the test itself sets a short
timeout for a command, we still want to give the test process enough
time to consume and log its output.
I also put some more thought into how one might debug a test hang, and
realized that in that case we don't want to set a WaitDelay at all:
instead, we want to leave the processes in their stuck state so that
they can be investigated with tools like `ps` and 'lsof'.