1. Working directory is not contaminated with results of the testsuite.
   Tests can be written in a more liberal way (creating/writing files
   etc).
2. The tests are more hermetic.. for example it was basically impossible
   to write a test which run outside of a project context.
3. The overall path lengths will be much shorter, which has been a
   consistent issue on windows. We can remove hacks like
   `withShorterPathForNewBuildStore`.

Fixes #9711