tests: fix SetupGnuPG test

The SetupGnuPG test tries to test the full setup, including the
creation of the directories. In order to do that, it create a
temporary directory, and redefines the home_dot_repo to point there.

When a home_dot_repo directory does not exist, it should be created.
The gpg_dir, which should exist inside home_dot_repo, also needs to be
created if it does not exist. However, since the gpg_dir path is
relative to home_dot_repo, once we redefine one, we need to redifine
the other.

The failure of this test might have gone unnoticed so far, since in
only fails if you do not have a ~/.repoconfig/gnupg/ on the
environment you are running the tests on.

Change-Id: Ic69d59e56137eea43349a61b5cf81f215c6a7f9a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/262573
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Marcos Marado <mindboosternoori@gmail.com>