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 #1338.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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