SystemNotificationManagerTest: Simplify test code

BUG=b:283709278
TEST=unit_tests --gtest_filter=SystemNotificationManagerTest.*

Change-Id: Ic93ce10e083aa3ae950259992078125bc8bc1eb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4572063
Auto-Submit: François Degros <fdegros@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Reviewed-by: Ben Reich <benreich@chromium.org>
Commit-Queue: Ben Reich <benreich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1150373}