system-test: clear cache; simpler init

* always run with a clean cache
 * in the future we should structure the tests so they run twice, once with a clean cache and then again with a populated one, but for now a clean cache should catch more bugs
* stop requiring a file cache path for tests; just use `test/tmp-file-cache`
* fix exit codes to match documentation at the top of the file: 1 is a test failure, 2 is bad args