linux: Fix failing exception handler server test
linux: Fix failing exception handler server test

The test broke at:
https://chromium-review.googlesource.com/c/crashpad/crashpad/+/978630
which moved responsibility for sending a ForkBroker request to
PtraceDecider. MockPtraceDecider wasn't updated to send this request,
so no broker was forked.

Change-Id: I8eddcc57c7b45419a72f1239c1cc9ab27e4ac2d2
Reviewed-on: https://chromium-review.googlesource.com/996715
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad