Fix checkstats test

[TimJentzsch]

The CI seems to be bugged and not always runs on PRs. Somehow, a buggy test got merged this way.

The api/tests/slack/commands/test_checkstats.py::test_check_stats_msg is failing:

E           Mod check stats for *<https://reddit.com/u/Moddington|u/Moddington>*:
E           
E           *Completed Checks*:
E           - All-time: 7 (58.3% of all checks)
E         - - Last 2 weeks: 4 (66.7% of all recent checks)
E         ?                 ^   ^ ^
E         + - Last 2 weeks: 3 (60.0% of all recent checks)
E         ?                 ^   ^ ^
E         - - Last completed: 2020-07-12 (1.6 weeks ago)
E         ?                            ^
E         + - Last completed: 2020-07-11 (1.6 weeks ago)
E         ?                            ^
E           
E           *Completed Warnings*:
E           - All-time: 1 (14.3% of checks, 50.0% of all warnings)
E           - Last 2 weeks: 0 (0.0% of recent checks, 0.0% of all recent warnings)
E         - - Last completed: 2020-07-05 (2.6 weeks ago)
E         ?                            ^
E         + - Last completed: 2020-07-04 (2.6 weeks ago)
E         ?                            ^
E           
E           *Completed Comments*:
E           - All-time: 3 (42.9% of checks, 75.0% of all comments)
E         - - Last 2 weeks: 2 (50.0% of recent checks, 100.0% of all recent comments)
E         ?                    ^^ ^
E         + - Last 2 weeks: 2 (66.7% of recent checks, 100.0% of all recent comments)
E         ?                    ^^ ^
E         - - Last completed: 2020-07-12 (1.6 weeks ago)
E         ?                            ^
E         + - Last completed: 2020-07-11 (1.6 weeks ago)
E         ?                            ^