tox: Make all tests timeout after 5min

Use pytest-timeout to make sure tests don't get stuck for more than
5 minutes. In future individual tests can exceed this timeout by
being decorated with @pytest.mark.timeout(600).

Signed-off-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Change-Id: I8f5b61a20230c22a86fd5636297c78f41369449a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/353124
Reviewed-by: Mike Frysinger <vapier@google.com>

tox.ini

@@ -26,9 +26,14 @@ python =
     3.10: py310
 
 [testenv]
-deps = pytest
+deps =
+    pytest
+    pytest-timeout
 commands = {envpython} run_tests
 setenv =
     GIT_AUTHOR_NAME = Repo test author
     GIT_COMMITTER_NAME = Repo test committer
     EMAIL = repo@gerrit.nodomain
+
+[pytest]
+timeout = 300