cmd/go: deflake TestScript/test2json_interrupt

- Start handling signals in 'go test' just before starting the test
  subprocess instead of just after. (It is unlikely that starting the
  process will cause cmd/go to hang in a way that requires signals to
  debug, and it is possible that something the test does — such as
  sending os.Interrupt to its parent processes — will immediately
  send a signal that needs to be handled.)

- In the test-test, don't try to re-parse the parent PIDs after
  sending signals, and sleep for a much shorter time interval.
  (Overrunning the sleep caused the next call to strconv.Atoi — which
  shouldn't even happen! — to fail with a parse error, leading to the
  failure mode observed in
  https://build.golang.org/log/f0982dcfc6a362f9c737eec3e7072dcc7ef29e32.)

Fixes #56083.
Updates #53563.

Change-Id: I346a95bdda5619632659ea854f98a9e17a6aede7
Reviewed-on: https://go-review.googlesource.com/c/go/+/456115
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

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