Add timeout for tests and remove macos test envs

[Li Jiang (thinkall)]

Why are these changes needed?

• In case some tests hang, make it timeout faster (2 hours instead of the default 6 hours).
• Remove macos test envs as it should be covered by ubuntu.

Related issue number

Checks

• I've used pre-commit to lint the changes in this PR (note the same in integrated in our CI checks).
• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
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