Option to build the test_build themes in CWD (and not tempdir)

[Carreau]

I think it would be useful to have an option/env variable to not erase the test rendering when running nox -s test, At least I wanted to check a few things and had a hard time figuring out how to get to those (e.g: checking there are enough menu items to have dropdowns).

I'm suggeting PST_BUILD_DIR='.' for example, and if set it would not erase the temp files as well as build in ..
Unless there is an option I have not found.

[trallard]

This should be part of #1759