fix: DB Seeder may use wrong DB connection during testing

[kenjis]

Description
The shared migration holds an old DB connection.
If SQLite3 :memory: is used, a new DB connection is a connection to a new database.

Before:

$ vendor/bin/phpunit tests/system/Database/Live/TransactionDBDebugTrueTest.php 
PHPUnit 9.6.16 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.15
Configuration: /Users/kenji/work/codeigniter/official/CodeIgniter4/phpunit.xml

.EEEE                                                                5 / 5 (100%)

Time: 00:00.136, Memory: 18.00 MB

There were 4 errors:

1) CodeIgniter\Database\Live\TransactionDBDebugTrueTest::testTransStartTransException
CodeIgniter\Database\Exceptions\DatabaseException: SQLite3::exec(): no such table: db_user

After:

$ vendor/bin/phpunit tests/system/Database/Live/TransactionDBDebugTrueTest.php 
PHPUnit 9.6.16 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.15
Configuration: /Users/kenji/work/codeigniter/official/CodeIgniter4/phpunit.xml

.....                                                                    5 / 5 (100%)

Time: 00:00.296, Memory: 18.00 MB

OK (5 tests, 16 assertions)

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