git-authenticate: Add tests.

git-authenticate: Add tests.

* guix/tests/git.scm (call-with-environment-variables)
(with-environment-variables): Remove.
* guix/tests/git.scm (populate-git-repository): Add clauses for signed
commits and signed merges.
* guix/tests/gnupg.scm: New file.
* tests/git-authenticate.scm: New file.
* tests/ed25519bis.key, tests/ed25519bis.sec: New files.
* Makefile.am (dist_noinst_DATA): Add 'guix/tests/gnupg.scm'.
(SCM_TESTS): Add 'tests/git-authenticate.scm'.
(EXTRA_DIST): Add tests/ed25519bis.{key,sec}.