adb: Improve test_adb a bit more

This change:

* uses unittest.main(), which allows for a subset of the tests to be
  selected.
* drops the requirement to have a device already connected since all the
  tests that need a device now spin their own mock device.
* Splits the monolithic test class into more granular classes.
* Makes this file be pylint-compliant.

Bug: None
Test: python system/core/adb/test_adb.py
Test: pylint system/core/adb/test_adb.py
Change-Id: I91c7ced520c3c69f855d639e0dbf7e57bb690e97