testenv: abstract run-with-timeout into testenv

This lifts the logic to run a subcommand with a timeout in a test from
the runtime's runTestProg into testenv. The implementation is
unchanged in this CL. We'll improve it in a future CL.

Currently, tests that run subcommands usually just timeout with no
useful output if the subcommand runs for too long. This is a step
toward improving this.

For #37405.

Change-Id: I2298770db516e216379c4c438e05d23cbbdda51d
Reviewed-on: https://go-review.googlesource.com/c/go/+/370701


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>