system_test nox sessions shouldn't ALWAYS "pass" with no config

[dhermes]

Was just chatting with @daspecster and running nox locally gives no real indication that the system tests did nothing.

Rather than checking for GOOGLE_APPLICATION_CREDENTIALS and saying "nothing to see here" we should probably instead do:

1. Bail without failing if in CI and not in a push build (e.g.)
2. If we've made it PAST the bail out (i.e. not on CI or in CI on a merge build) than FAIL THE TEST if GOOGLE_APPLICATION_CREDENTIALS is unset

[lukesneeringer]

running nox locally gives no real indication that the system tests did nothing.

Running nox locally should run system tests. That is a bug. I will fix it.
Your list sounds correct.

[lukesneeringer]

Superceded by #3475.