os/exec: parallelize more tests

os/exec: parallelize more tests

This cuts the wall duration for 'go test os/exec' and
'go test -race os/exec' roughly in half on my machine,
which is an even more significant speedup with a high '-count'.

For better or for worse, it may also increase the repro rate
of #34988.

Tests that use Setenv or Chdir or check for FDs opened during the test
still cannot be parallelized, but they are only a few of those.

Change-Id: I8d284d8bff05787853f825ef144aeb7a4126847f
Reviewed-on: https://go-review.googlesource.com/c/go/+/439196
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>