runtime: reduce timing sensitivity in TestEINTR
runtime: reduce timing sensitivity in TestEINTR

- Don't assume that a process interrupted at 100μs intervals will have
  enough remaining time to make progress. (Stop sending signals
  in between signal storms to allow the process to quiesce.)

- Don't assume that a child process that spins for 1ms will block long
  enough for the parent process to receive signals or make meaningful
  progress. (Instead, have the child block indefinitely, and unblock
  it explicitly after the signal storm.)

For #39043
Updates #22838
Updates #20400

Change-Id: I85cba23498c346a637e6cfe8684ca0c478562a93
Reviewed-on: https://go-review.googlesource.com/c/go/+/233877
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,cloudflare/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,golang/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,CAFxX/go,nawawi/go,golang/go,akutz/go,cloudflare/go,cloudflare/go