fix unit tests by not cleaning up all $EASYBUILD_X env vars, only the ones that matter

@pforai, @pescobar: are you OK with this, bumping the required Lmod version to the just released v5.6.3 (for the upcoming EasyBuild v1.14.0)? There are a couple of critical fixes in that version, on which the current `develop` version of framework depends (depending on the configuration, but doing different version checks based on how EB is configured is not something I want to get into)