fix py312 warning for `datetime.utcnow`

[waketzheng]

Description

Actions log of Run ci with Python3.12 print the following warning:

tests/fields/test_time.py::TestDatetimeFields::test_get_utcnow
  /home/runner/work/tortoise-orm/tortoise-orm/tests/fields/test_time.py:78: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    now = datetime.utcnow().replace(tzinfo=get_default_timezone())

Motivation and Context

While fixing the warning above, I found that test_time.py can be refactor to reduce duplicated code.

How Has This Been Tested?

make ci

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added the changelog accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 9564975579

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.049%

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Totals
Change from base Build 9563237685:	0.0%
Covered Lines:	5868
Relevant Lines:	6492

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💛 - Coveralls