Added more argument validation for the user-switch event.

Added functionaility for further validation of the --event,
--from-user, and --to-user arguments in torq.py. This is
necessary to make sure invalid user-switch runs are not
attempted. Furthermore, integrated these changes into
tests/torq_unit_test.py by slightly modifying an existing
unit test and adding 4 new unit tests.

Test: atest torq_unit_test
Bug: 356402331

Change-Id: I157ad2c6344e552cd7296527402898df075ba1ea
2 files changed