Add test util for checking stand-alone python scripts

[innerlee]

This function can be used to check stand-alone scripts, without tweaking sys.path manually. Moreover, it will increase test coverage for codes the script touches.

[codecov]

Codecov Report

Merging #1007 (a1d104b) into master (c4aa414) will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #1007      +/-   ##
==========================================
- Coverage   64.60%   64.58%   -0.02%     
==========================================
  Files         152      152              
  Lines        9779     9802      +23     
  Branches     1779     1780       +1     
==========================================
+ Hits         6318     6331      +13     
- Misses       3133     3142       +9     
- Partials      328      329       +1     

Flag	Coverage Δ
unittests	64.58% <80.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
mmcv/utils/__init__.py	88.23% <ø> (ø)
mmcv/utils/testing.py	96.00% <80.00%> (-4.00%)	⬇️
mmcv/ops/corner_pool.py	52.77% <0.00%> (-3.16%)	⬇️

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