chore: add pull request template

[xushiyan]

Description

Add PR template to emphasize on testing.

How are the changes test-covered

• [x] N/A
• [ ] Automated tests (unit and/or integration tests)
• [ ] Manual tests
  • [ ] Details are described below

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.24%. Comparing base (36316e0) to head (034a54d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #89 +/- ## ======================================= Coverage 87.24% 87.24% ======================================= Files 14 14 Lines 690 690 ======================================= Hits 602 602 Misses 88 88 ```

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