Migrated NestField tests to @testing-library/react

[piotrpospiech]

#1346

• Migrated MUI NestField tests to RTL
• Material NestField tests weren't changed. They will be removed entirely in the Deprecate uniforms-material theme #1338.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.35%. Comparing base (a8f4ced) to head (8cb0f7e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1350      +/-   ##
==========================================
+ Coverage   94.34%   94.35%   +0.01%     
==========================================
  Files         200      200              
  Lines        3306     3314       +8     
  Branches      896      895       -1     
==========================================
+ Hits         3119     3127       +8     
  Misses         70       70              
  Partials      117      117              

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