Don't use exclude newer manually in test functions (#2697)
With this change, all usages of `EXCLUDE_NEWER` are now in command
wrappers, not in the test functions themselves.
For the venv test, i refactored them into the same kind of test context
abstraction that the other test modules have in the second commit.
The third commit makes`"INSTA_FILTERS` "private", removing the last
remaining individual usage.
Pending windows CI :crossed_fingers:
