[testing] move pytest to be inside the function

[FrankLeeeee]

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Fixed #3962

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This PR moved import pytest to the function body so that it will not affect users who use the CLI.

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[github-actions]

The code coverage for the changed files is 75%.

Click me to view the complete report

Name                                   Stmts   Miss  Cover
----------------------------------------------------------
colossalai/testing/pytest_wrapper.py      12      3    75%
----------------------------------------------------------
TOTAL                                     12      3    75%