Clean-up: Test list triggers: Remove unused tmp variables

Clean-up: Test list triggers: Remove unused tmp variables

The variables can be removed since the functions that compare the
actual and expected output for these tests, `list_triggers_matches_ok`
and `list_triggers_matches_mi_ok` define `tmp_stdout` and `tmp_stderr`
variables locally.

Change-Id: Ifa54a4034f4b6ed6ed0a1676f7c9e36d1c9affc6
Signed-off-by: Erica Bugden <ebugden@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>