Deflake Pdfork.PdkillOtherSignal test
Deflake Pdfork.PdkillOtherSignal test

This test would occasionally flake; best guess is that the
signal arrives just before the long sleep, and the call
to EXPECT_PID_DEAD doesn't wait long enough to catch it.
So reduce the long sleep in the loop in the child.

google/capsicum-test,google/capsicum-test,google/capsicum-test,google/capsicum-test