Increase coverage in jest-diff

[pedrottimark]

Summary

Before designing new tests for char diff:

• cover last uncovered line in diff_strings.js
• cover one additional line in index.js but leave 6 related to asymmetric matcher

In diff.test.js

• Factored out const NO_DIFF_MESSAGE = 'Compared values have no visual difference.';
• Moved toJSON function from module scope to describe block where it is used

Test plan

• Added unexpanded tests under no visual difference to hit if (hunks.length === 0) { return null; } in diff_strings.js
• Deleted one test under oneline strings that is redundant with no visual difference
• Added tests under refactored falls back to not call toJSON to hit catch (e) { hasThrown = true; } in index.js

[codecov-io]

Codecov Report

Merging #4583 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4583      +/-   ##
==========================================
+ Coverage   55.65%   55.68%   +0.03%     
==========================================
  Files         186      186              
  Lines        6348     6348              
  Branches        3        3              
==========================================
+ Hits         3533     3535       +2     
+ Misses       2814     2812       -2     
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-diff/src/diff_strings.js	100% <0%> (+1.16%)	⬆️
packages/jest-diff/src/index.js	84.09% <0%> (+2.27%)	⬆️

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[github-actions]

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