crash-reporter: Clean up testing code.

Link all of the crash reporter's tests into a single
crash_reporter_test executable instead of building separate
binaries that lead to a bunch of duplication.

BUG=none
TEST=tests pass
CQ-DEPEND=I7f6623dd0ae36395efac89bdcfae54926ea6c918

Change-Id: I18e2d929d6545e7d05e88232bdaec089ae620544
Reviewed-on: https://chromium-review.googlesource.com/243940


Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Dan Erat <derat@chromium.org>
Trybot-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>