test: use short classname for config()

[kenjis]

Follow-up #796
Ref #802

This PR fixes the following test error with CI v4.4.0:

1) Tests\Unit\Authentication\JWT\JWTManagerTest::testParseCanSpecifyKey
ErrorException: Undefined array key "mobile"

/home/runner/work/shield/shield/src/Authentication/JWT/Adapters/FirebaseAdapter.php:145
/home/runner/work/shield/shield/src/Authentication/JWT/Adapters/FirebaseAdapter.php:112
/home/runner/work/shield/shield/src/Authentication/JWT/JWSEncoder.php:62
/home/runner/work/shield/shield/src/Authentication/JWTManager.php:[73](https://github.com/codeigniter4/shield/actions/runs/5984579479/job/16235976345?pr=801#step:11:74)
/home/runner/work/shield/shield/tests/Unit/Authentication/JWT/JWTManagerTest.php:311

[kenjis]

I just changed the commit message.
This PR is no problem, but it seems to be better to fix the config() behavior.