Sanitize tests environment.

* tests/Makefile.am (clean-local): New target.
(TESTS_ENVIRONMENT): New variable.
(EXTRA_DIST): New variable.
* tests/clean-socketdir: New file.

We should avoid using the current user's GNUPGHOME directory
(and any pre-existing agent) when running the tests.

Then, since running the tests imply spawning an agent, we should
wipe out the corresponding socket directory upon 'make clean'.
This will also terminate the agent.

• GnuPG-bug-id: T3274
• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>