util-core: improve test coverage for Cancellable

[muachilin]

Problem

The test coverage of Cancellable still needs to be improved. Particularly the object of Cancellable has not been tested.

Solution

Add more tests for Cancellable.

Result

Improve the test coverage for Cancellable.

[muachilin]

Hello @ryanoneill, I created this pull request with only the file changes for testing Cancellable. Would you please check whether the patch is clean or not? Thank you very much!

[codecov-io]

Codecov Report

Merging #270 into develop will increase coverage by 0.06%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #270      +/-   ##
===========================================
+ Coverage       46%   46.07%   +0.06%     
===========================================
  Files          228      228              
  Lines        14443    14443              
  Branches       889      922      +33     
===========================================
+ Hits          6645     6654       +9     
+ Misses        7798     7789       -9

Impacted Files	Coverage Δ
...core/src/main/scala/com/twitter/util/Promise.scala	77.95% <0%> (+0.4%)	:arrow_up:
...in/scala/com/twitter/logging/QueueingHandler.scala	100% <0%> (+6.06%)	:arrow_up:
.../src/main/scala/com/twitter/util/Cancellable.scala	100% <0%> (+75%)	:arrow_up:

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[ryanoneill]

Thank you. I'm handling this internally now. I'll post back when the commit has been pushed back to GitHub.

[ryanoneill]

Thanks again @muachilin. This was pushed back out to GitHub as commit https://github.com/twitter/util/commit/41227fa84087375ae8d1def4c04eadaba64f30e5

[muachilin]

Thank you :)