Devil: delete ANDROID_SERIAL before running tests
Devil: delete ANDROID_SERIAL before running tests

Devil unit tests tend to mock out internal details of other methods.
Since ANDROID_SERIAL can affect which internal code paths are used,
setting this environmental variable can break these assumptions and
trigger test failures. To simplify expectations for tests, delete this
variable before starting the test runner.

Bug: chromium:955155
Test: ANDROID_SERIAL=emulator-5554 ./devil/bin/run_py_tests
Change-Id: Ic7b4727d8df9274488483ba7266527b312ce1f01
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/1707528
Reviewed-by: Ben Pastene <e214258550f76fa23ac7c1d953424f3f468f5c6b@chromium.org>
Commit-Queue: Nate Fischer <72afde956856aeb0f00e5e8df797f0068a2ecaf8@chromium.org>

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