[test] refactor tests with spawn

[FrankLeeeee]

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Fixed #3458

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This PR refactored the test-related code into the colossalai.testing module.

• moved free_port to colossalai.testing as it is only used in testing
• implemented the spawn function to automatically spawn a function with the same port
• implemented the clear_cache_before_run decorator to lower the memory usage

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[FrankLeeeee]

These two tests failed due to network error, I have tested them locally.