[Enhance] Add runner test case.

[HAOCHENYE]

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Motivation

All unit tests need to build runner could inherit TestRunnerTestCase.

Modification

1. A lot of unit test needs to build a Runner instance, RunnerTestCase provides a simple way to build runner with iter based or epoch-based config.
2. Building a Runner will leave a lot of global variable, RunnerTestCase will clean then after each unit test.
3. RunnerTestCase will deinitialize the distributed enviroment.
4. Registry instance like MODELS and HOOKS are also global variables, RunnerTestCase will also clean them after each unit test,

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Base: 78.45% // Head: 78.58% // Increases project coverage by +0.12% 🎉

Coverage data is based on head (89cdaaa) compared to base (c144773).
Patch coverage: 93.06% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #631      +/-   ##
==========================================
+ Coverage   78.45%   78.58%   +0.12%     
==========================================
  Files         126      128       +2     
  Lines        9141     9283     +142     
  Branches     1818     1834      +16     
==========================================
+ Hits         7172     7295     +123     
- Misses       1656     1672      +16     
- Partials      313      316       +3     

Flag	Coverage Δ
unittests	78.58% <93.06%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmengine/testing/runner_test_case.py	92.92% <92.92%> (ø)
mmengine/model/wrappers/utils.py	100.00% <100.00%> (+10.00%)	⬆️
mmengine/testing/__init__.py	100.00% <100.00%> (ø)
mmengine/dataset/dataset_wrapper.py	94.59% <0.00%> (-0.70%)	⬇️
mmengine/runner/checkpoint.py	43.95% <0.00%> (-0.68%)	⬇️
mmengine/hooks/checkpoint_hook.py	89.79% <0.00%> (-0.52%)	⬇️
mmengine/config/utils.py	58.49% <0.00%> (ø)
mmengine/runner/runner.py	84.68% <0.00%> (ø)
mmengine/hooks/ema_hook.py	93.33% <0.00%> (ø)
mmengine/utils/__init__.py	100.00% <0.00%> (ø)
... and 9 more

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