Previously, tests ran in 5.7s, without cleanup, now it takes 8.5s,
a reduction by 1/3rd which is noticable (removing 15k files per
full run takes time).
When setting the GITBUTLER_TESTS_NO_CLEANUP environment variable
when running the tests, temporary directories won't be removed
anymore.
Created on the fly while perusing the test-suite.
* avoid unnecessary `test_` prefix
* move more (and previously missed) test modules to integration level
* make `dedup` crate-private to have a reason to keep the tests where they are, same with similar functions
