Improve test coverage IdT

[peterneyens]

I am not sure how often IdT is actually used, but this PR should improve its test coverage.

I think it makes sense to merge #1621 first, I will replace TransLift with MonadTrans in this PR afterwards.

[codecov-io]

Codecov Report

Merging #1623 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1623      +/-   ##
==========================================
+ Coverage   93.88%   94.06%   +0.17%     
==========================================
  Files         246      246              
  Lines        4088     4094       +6     
  Branches      154      160       +6     
==========================================
+ Hits         3838     3851      +13     
+ Misses        250      243       -7

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/IdT.scala	96.66% <100%> (+25.83%)	⬆️
...rc/main/scala/cats/laws/discipline/Arbitrary.scala	92.06% <0%> (+1.58%)	⬆️

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[peterneyens]

Updated to use MonadTrans and added some missing serializable tests for MonadTrans instances.

[kailuowang]

👍