Convert tests to in memory

[rklaehn]

There are a large number of tests which use files which are not meant to test file IO.

They can all be replaced with tests that work with an in memory database. Maybe that helps with test execution times.

Look for this:

Path::new(env!("CARGO_MANIFEST_DIR")).join("README.md");

[rklaehn]

Implemented in #1232

[dignifiedquire]

can we close this?

[rklaehn]

I think most tests are now in mem. We don't want them all to be in mem, otherwise we would never test the file io stuff. So I think we can close this.