chore(slack): Update Tests for `get_channel_id`

[iamrajjoshi]

I GA'ed the feature, so I am updating the tests here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.12%. Comparing base (81bbf5d) to head (9b57418). Report is 25 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #73548       +/-   ##
===========================================
+ Coverage   56.95%   78.12%   +21.17%     
===========================================
  Files        6656     6665        +9     
  Lines      297619   297885      +266     
  Branches    51231    51268       +37     
===========================================
+ Hits       169505   232725    +63220     
+ Misses     123389    58892    -64497     
- Partials     4725     6268     +1543     

Files	Coverage Δ
src/sentry/integrations/slack/utils/channel.py	86.66% <100.00%> (+65.28%)	:arrow_up:
...sks/integrations/slack/find_channel_id_for_rule.py	70.49% <100.00%> (+42.36%)	:arrow_up:

... and 2007 files with indirect coverage changes