default to not collecting test data

[g-k]

Currently we have kind of insane --ignore strings, which is confusing and defaults to frost trying to slowly collect resources on each run. Proposal:

• upgrade to pytest 4.3.0+ https://docs.pytest.org/en/stable/changelog.html#pytest-4-3-0-2019-02-16 (and dependent plugins)
• add collect_ignore_glob to conftest.py https://docs.pytest.org/en/stable/reference.html#globalvar-collect_ignore_glob
• update docs to remove --ignore strings and require explicit test inclusion