test: fix flaky gh-2717-no-quit-sigint

This test is quite a flaky in debug ASAN build. Let's fix it before
turning debug ASAN on in CI.

The issue is due to heavy load popen.read may return nil with 'TimedOut:
timed out' error. Just read again as in the other cases of this test.

Part of #7327

NO_CHANGELOG=internal
NO_DOC=internal

(cherry picked from commit 6f48b8d7)