test: add tests for Model timestamp

[kenjis]

Description

• add tests for the current behaviors.

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide

[kenjis]

Only SQLSRV returns datetime with milliseconds.

1) CodeIgniter\Models\TimestampModelTest::testDoNotAllowDatesInsertArrayWithoutDatesSetsTimestamp
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'2023-11-25 12:00:00'
+'2023-11-25 12:00:00.000'

/home/runner/work/CodeIgniter4/CodeIgniter4/tests/system/Models/TimestampModelTest.php:93
phpvfscomposer:///home/runner/work/CodeIgniter4/CodeIgniter4/vendor/phpunit/phpunit/phpunit:106

https://github.com/codeigniter4/CodeIgniter4/actions/runs/6987412341/job/19014063118?pr=8249