Fix errors when identity-obj-proxy mocks CSS Modules

[pedrottimark]

Summary

Fixes #4580 and #4922

Because the mock identity object for CSS Modules returns the key as the value for any key:

• Comparing sentinel properties to true avoids false positive in Immutable plugin test method
• Comparing val.constructor === 'function' avoids undefined as name of object

See Mocking CSS Modules

Test plan

Added 2 tests which failed before changes to code, and then fixed them to fail for right reason :)

[codecov-io]

Codecov Report

Merging #4935 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4935      +/-   ##
==========================================
+ Coverage   60.26%   60.27%   +0.01%     
==========================================
  Files         198      198              
  Lines        6593     6595       +2     
  Branches        3        3              
==========================================
+ Hits         3973     3975       +2     
  Misses       2620     2620

Impacted Files	Coverage Δ
packages/pretty-format/src/index.js	96.45% <100%> (+0.05%)	⬆️
packages/pretty-format/src/plugins/immutable.js	100% <100%> (ø)	⬆️

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[SimenB]

Nice and clean 🙂

[github-actions]

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