Improve test coverage, especially around caches.

[non]

We still have coverage gaps in Rule.scala and Dag.scala, which will
have to be dealt with another time.

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 3.75%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #12      +/-   ##
=======================================
+ Coverage   86.24%   90%   +3.75%     
=======================================
  Files          12    12              
  Lines         189   190       +1     
  Branches       15    15              
=======================================
+ Hits          163   171       +8     
+ Misses         26    19       -7

Impacted Files	Coverage Δ
core/src/main/scala/com/stripe/dagon/HMap.scala	95.45% <100%> (+4.97%)	⬆️
core/src/main/scala/com/stripe/dagon/HCache.scala	100% <0%> (+42.85%)	⬆️
core/src/main/scala/com/stripe/dagon/Cache.scala	100% <0%> (+42.85%)	⬆️

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[johnynek]

👍

Thanks for the coverage.