core/manager: when running in test mode, use a temp dir for generated stuff

core/manager: when running in test mode, use a temp dir for generated stuff

When running through systemd-analyze verify or with --test, we would
not run generators (environment or unit). But at the end, we would nuke
the generator dirs anyway.

Simplify things by actually running generators of both types, but redirecting
their output to a temporary directory. This has the advantage that we test more
code, and the verification is more complete.

Since now we are not touching the real generator directories, we also don't
delete them, which fixes #5609.