os/exec: in TestContextCancel, dump goroutines on failure

If this test fails, we want to know exactly what the os/exec
goroutines are doing. Panicking gives us a goroutine dump,
whereas t.Fatal does not.

While we're here, use exponential backoff instead of a hard-coded 1ms
sleep. We want to give the OS enough time to actually terminate the
subprocess.

For #42061

Change-Id: I3d50a71ac314853c68a935218e7f97ce18b08b5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/368317


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>