Explicit auth tests

[max-sixty]

After I neglected #127 and @tswast valiantly completed the bulk, there were a couple of useful unrelated pieces that are also worth adding

Here, tests are explicitly marked with auth type so, for example, if pytest -m 'not s_path_auth' is run, it won't run any tests with service path auth
Travis is then parameterized by the command it's passed, rather than the tests themselves checking whether they're running in travis

[tswast]

Thanks!

[tswast]

Looks like unit tests got included twice.

[tswast]

It seems like test_get_application_default_credentials_does_not_throw_error and test_get_application_default_credentials_returns_credentials are working just fine on Travis if you want to move this to the test_get_user_account_credentials_XXX methods.

[tswast]

This doesn't seem to be skipping local auth tests no matter where I put the mark. :-(

$ nox -s default -r | grep TestAuth
nox > Running session default
nox > Re-using existing virtualenv at /Users/swast/src/pandas/pandas-gbq/.nox/default.
nox > chdir /Users/swast/src/pandas/pandas-gbq
nox > Skipping installation of "mock pytest pytest-cov".
nox > Skipping installation of "-e .".
nox > pytest ./tests/unit ./tests/system.py -m 'not local_auth and not s_path_auth' -v --cov=pandas_gbq --cov=tests.unit --cov-report xml:/tmp/pytest-cov.xml
tests/system.py::TestAuth::test_get_application_default_credentials_does_not_throw_error[env] PASSED [ 33%]
tests/system.py::TestAuth::test_get_application_default_credentials_returns_credentials[env] PASSED [ 33%]
tests/system.py::TestAuth::test_get_user_account_credentials_bad_file_returns_credentials[env] PASSED [ 34%]
tests/system.py::TestAuth::test_get_user_account_credentials_returns_credentials[env] PASSED [ 35%]
^Cnox > Interrupted...
nox > Session default interrupted.

[max-sixty]

Discovery, though not solution - if there's a legacy-style autouse=True fixture, even the explicit marking (like you added) doesn't work.

We could try and resolve, or move to new-style tests (which would be preferable, but non-zero work)

[tswast]

Ugh. Okay, well I've wanted to move to new-style tests for a while. (Already did it with the unit tests.) I'll see if I can get a PR to refactor the system tests out today.

[tswast]

Well, I only got to the auth tests, but it's a start. #176

[tswast]

Closing in favor of #176 changes.