Fix env. variable type issue in testing
# What does this PR do?

Fix env. variable type issue in testing.

If `PYTEST_TIMEOUT` is set by `export PYTEST_TIMEOUT=...` or `PYTEST_TIMEOUT=xxx python3 -m pytest ...`, we actually get a `string` instead of `int`, and the test fails.

On our CI (within docker), we don't have this issue, probably due to the way of docker dealing with env. variable.
Let's try to avoid unexpected error though.


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