more broker tests

[travisjeffery]

• test shutdown
• test running leader and isr request for a new partition

[codecov]

Codecov Report

Merging #77 into master will increase coverage by 2.39%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   59.52%   61.91%   +2.39%     
==========================================
  Files          17       17              
  Lines        1586     1586              
==========================================
+ Hits          944      982      +38     
+ Misses        545      499      -46     
- Partials       97      105       +8

Impacted Files	Coverage Δ
broker/broker.go	66.41% <50%> (+6.75%)	⬆️
broker/fsm.go	29.78% <0%> (+4.25%)	⬆️

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