For most tests, the test's deadline itself is more appropriate than an
arbitrary timeout layered atop of it (especially once #48157 is
implemented), and testenv.Command already adds cleaner timeout
behavior when a command would run too close to the test's deadline.
That makes RunWithTimeout something of an attractive nuisance. For
now, migrate the two existing uses of it to testenv.CommandContext,
with a shorter timeout implemented using context.WithTimeout.
As a followup, we may want to drop the extra timeouts from these
invocations entirely.