Adding no-cover pragmas to unittest.main().

This is so we get 100% line coverage in our tests. Also
adding unittest.main() lines to the files missing it:

test__helpers.py
test__pycrypto_crypt.py
test_crypt.py
test_devshell.py
test_gce.py
test_keyring_storage.py
test_service_account.py
test_util.py