Clean up files generated by make check.

* tests/clean-socketdir: Remove more test-generated files.

Running make check may generate more files in the test directory
(notably, if the tests are run while a token is present, the agent
will have written down the shadow keys), that we need to cleanup
afterwards.

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