Minor cleanup to some tests that register custom config

[famod]

As discussed here: #22821 (comment)

Btw, that superfluous catch also exists in some main classes (e.g. ExtensionLoader), but I didn't want to touch those.

[quarkus-bot]

This workflow status is outdated as a new workflow run has been triggered.

Failing Jobs - Building c127043

Status	Name	Step	Failures	Logs	Raw logs
✖	Initial JDK 11 Build	Build	Failures	Logs	Raw logs

Failures

⚙️ Initial JDK 11 Build #

- Failing: core/runtime 
! Skipped: core/deployment core/test-extension/deployment core/test-extension/runtime and 674 more

📦 core/runtime

✖ Failed to execute goal net.revelc.code:impsort-maven-plugin:1.6.2:check (check-imports) on project quarkus-core: Imports are not sorted in /home/runner/work/quarkus/quarkus/core/runtime/src/test/java/io/quarkus/runtime/configuration/DotEnvTestCase.java

[radcortez]

Thanks for the PR @famod :)

[famod]

Yw, thanks for reviewing!