arc: ArcAdbdMonitorBridgeTest: Check environment passed to Upstart job

ArcAdbdMonitorBridge is responsible for listening to AdbdStarted /
AdbdStopped signals (implemented as mojo methods) from ARC and starting
/ stopping arcvm-adbd Upstart jobs with appropriate environment
variables fetched from ArcSessionManager.
Currently, ArcAdbdMonitorBridgeTest only checks that the jobs are
correctly started or stopped. This CL extends the test to also check the
environment variables passed to the Upstart job when starting them.

BUG=b:305608912
TEST=unit_tests --gtest_filter="ArcAdbdMonitorBridgeTest.*"

Change-Id: I92ee153c51d349c5587a074db085f2f56846dd8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4942550
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Youkichi Hosoi <youkichihosoi@chromium.org>
Commit-Queue: Momoko Hattori <momohatt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1212657}