[branch-4.1](regression) Restore time zone after test_tz_load

[shuke987]

Summary

• restore the original global time_zone after test_tz_load changes it during the suite
• clean up the temporary global_timezone_test table in the finally block
• prevent later nonConcurrent suites from inheriting UTC global time zone

Testing

• Verified @@global.time_zone and @@time_zone are Asia/Shanghai after the runs
• Buildall skipped per request

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[shuke987]

skip buildall