Use the root locale during the validation tests

[MaisiKoleni]

The problem messages that get checked in the validation tests are locale-sensitive and therefore, the tests fail on some computers with non-English default locale, e.g., German or French. Setting and restoring the root locale should fix the problem without interfering with other tests / steps in the build process.

Note that I chose to add this directly into the test class such that it works in the IDE without running Maven, too.

Some alternatives would be to extract the logic into a super class or JUnit 5 test extension, but this is the only test in this repository with the problem.

Example errors:

// German locale
org.opentest4j.AssertionFailedError: expected: <server.port must be greater than or equal to 8000> but was: <server.port muss größer-gleich 8000 sein>
// French locale
org.opentest4j.AssertionFailedError: expected: <server.port must be greater than or equal to 8000> but was: <server.port doit être supérieur ou égal à 8000>

[radcortez]

Thank you :)