os/exec: eliminate some arbitrary short timeouts

os/exec: eliminate some arbitrary short timeouts

These tests appear to be using timeouts to check for deadlocks or to
cause the test to fail earlier. However, on slower machines these
short timeouts can cause spurious failures, and even on faster
machines if the test locks up we usually want a goroutine dump instead
of a short failure message anyway.

Fixes #52818 (maybe).

Change-Id: Ib8f18d679f9443721e8a924caef6dc8d214fca1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/405434
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>