Fix CreateDatabase leaving altered database config in connection

[paulbalandan]

Description
When running vendor/bin/phpunit --group DatabaseLive either on GA or local, MigrationRunnerTest fails:

On GA:

PHPUnit 9.5.26 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.4.32
Configuration: /home/runner/work/CodeIgniter4/CodeIgniter4/phpunit.xml.dist

..S............................................................  63 / 516 ( 12%)
............S.S...S...........S................................ 126 / 516 ( 24%)
...................................................SSSSSSS..... 189 / 516 ( 36%)
............................................................... 252 / 516 ( 48%)
.............F................................................. 315 / 516 ( 61%)
............................................................... 378 / 516 ( 73%)
............................................................... 441 / 516 ( 85%)
..........SSSSSSSSSSSS......................................... 504 / 516 ( 97%)
............                                                    516 / 516 (100%)

Time: 00:[20](https://github.com/codeigniter4/CodeIgniter4/actions/runs/3456144117/jobs/5768709007#step:11:21).493, Memory: 102.00 MB

There was 1 failure:

1) CodeIgniter\Database\Migrations\MigrationRunnerTest::testLoadsDefaultDatabaseWhenNoneSpecified
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'/home/runner/work/CodeIgniter4/CodeIgniter4/writable/database.db'
+'/home/runner/work/CodeIgniter4/CodeIgniter4/writable/foobar.db'

/home/runner/work/CodeIgniter4/CodeIgniter4/tests/system/Database/Migrations/MigrationRunnerTest.php:89
phpvfscomposer:///home/runner/work/CodeIgniter4/CodeIgniter4/vendor/phpunit/phpunit/phpunit:97

On local: (forgive the irrelevant failures)

PHPUnit 9.5.26 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.1.12
Configuration: C:\Users\P\Desktop\Web Dev\CodeIgniter4\phpunit.xml.dist

..S.....FFFF.FFFF..........................................................S.S...S...........S.................. 112 / 516 ( 21%)
.................................................................SSSSSSS........................................ 224 / 516 ( 43%)
.........................................F...................................................................... 336 / 516 ( 65%)
................................................................................................................ 448 / 516 ( 86%)
...SSSSSSSSSSSSSSSSSSS..............................................                                             516 / 516 (100%)

Nexus\PHPUnit\Extension\Tachycardia identified this sole slow test:
⚠  Took 1.86s from 0.50s limit to run CodeIgniter\\Database\\Live\\GroupTest::testOrNotHavingLike


Time: 01:20.643, Memory: 108.00 MB

There were 9 failures:

...

9) CodeIgniter\Database\Migrations\MigrationRunnerTest::testLoadsDefaultDatabaseWhenNoneSpecified
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-':memory:'
+'C:\Users\P\Desktop\Web Dev\CodeIgniter4\writable\foobar.db'

After this PR:
In local:

PHPUnit 9.5.26 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.1.12
Configuration: C:\Users\P\Desktop\Web Dev\CodeIgniter4\phpunit.xml.dist

..S.....FFFF.FFFF..........................................................S.S...S...........S.................. 112 / 516 ( 21%)
.................................................................SSSSSSS........................................ 224 / 516 ( 43%)
................................................................................................................ 336 / 516 ( 65%)
................................................................................................................ 448 / 516 ( 86%)
...SSSSSSSSSSSSSSSSSSS..............................................                                             516 / 516 (100%)

Time: 00:13.382, Memory: 108.00 MB

The reason for this is that all database live tests uses the shared DB connection (from Database::connect()). When CreateDatabaseTest runs for SQLite3, the tests alter the database to use foobar.db in the Config\Database instance. On cleanup, the altered config is reset. However, the protected Database::$instances array contains the shared DB connection which still uses the altered config. Running Database::connect(null, false) will effectively flush the old shared connection and create a new one using the fresh config.

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

[MGatner]

Thanks for the investigation and thorough explanation! Glad to see one more of these local-versus-pipeline mysteries cleared up.