types: Reduce size of TestImmutability<Arith\U>Int tests

[ValarDragon]

Currently they take ~1 minute on circle CI. There isn't a significant reason
for this to delay all our tests. This commit reduces the time spent on these
tests by a factor of 20.

• Added entries in PENDING.md that include links to the relevant issue or PR that most accurately describes the change. - not user facing

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[ValarDragon]

Now the time spent in types is ok github.com/cosmos/cosmos-sdk/types 4.303s coverage: 82.6% of statements, way better than the previous 1 minute :)

[codecov]

Codecov Report

Merging #1936 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1936   +/-   ##
========================================
  Coverage    64.71%   64.71%           
========================================
  Files          114      114           
  Lines         6782     6782           
========================================
  Hits          4389     4389           
  Misses        2114     2114           
  Partials       279      279

[cwgoes]

utACK, thanks!