migrate tests to midje, add tests, code cleanup, small idiomatic changes to src

[gkspranger]

• this is for YB.core.adapters.slack and its related tests
• please double check my changes to SRC from (dorun (map)) to (run!) << trying to make codeclimate happy
• we don't have any tests in place for these given they are primarily side-effects, and while my local testing seems AOK, good to have extra eyes on this to make sure i am not missing something important

[codecov[bot]]

Codecov Report

Merging #176 (100950e) into master (4816091) will increase coverage by 0.43%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   49.95%   50.38%   +0.43%     
==========================================
  Files          94       94              
  Lines        4132     4132              
  Branches      196      195       -1     
==========================================
+ Hits         2064     2082      +18     
+ Misses       1872     1855      -17     
+ Partials      196      195       -1     

Impacted Files	Coverage Δ
src/yetibot/core/adapters/slack.clj	21.15% <0.00%> (+5.21%)	:arrow_up:

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