[Fix] Deprecate the support for "python setup.py test"

[fangyixiao18]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Update the setup.py
Refer to open-mmlab/mmcv#1637

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
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

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #174 (b5f607c) into master (f06c3c3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #174   +/-   ##
=======================================
  Coverage   59.96%   59.96%           
=======================================
  Files          94       94           
  Lines        3387     3387           
  Branches      542      542           
=======================================
  Hits         2031     2031           
  Misses       1271     1271           
  Partials       85       85           

Flag	Coverage Δ
unittests	59.96% <ø> (ø)

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