Refactor systests into separate modules, using pytest fixtures / idioms

[tseaver]

See #438, #439, ..., #469, #470.

The current setup / teardown code in tests/system/ is ancient, creaky, and not hardened well against quota restrictions / throttling.

• Split the tests/system/test_system.py monolith up into more focused modules.
• Re-do its setup / teardown using pytest fixtures, taking care to harden against 429 / 503 errors.
• Likewise for tests/system/tests_system_dbapi.py.