adb: add `adb shell exit 42` stress test.

Add a test to hammer on `adb shell exit $n` for flakiness.

Bug: http://b/74616284
Test: python test_device.py
Change-Id: I6a842960f5b55ff739044698f5c9683992fc42f1
(cherry picked from commit c970aefadace2203f23f0b028b6c6be16e55dd04)
1 file changed