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

[mzr1996]

Motivation

Refers to open-mmlab/mmcv#1637

Modification

Update the setup.py

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

❗ No coverage uploaded for pull request base (dev@5232965). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             dev     #646   +/-   ##
======================================
  Coverage       ?   82.12%           
======================================
  Files          ?      119           
  Lines          ?     6865           
  Branches       ?     1184           
======================================
  Hits           ?     5638           
  Misses         ?     1063           
  Partials       ?      164           

Flag	Coverage Δ
unittests	82.12% <0.00%> (?)

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