Add integration test to `log4j-samples-jlink`

Similarly to what we already have in `log4j-samples-graalvm`, the `log4j-samples-jlink` module should have an integration test that verifies that the `Main` class works correctly using the custom JLink environment (see [`StandardIT`](https://github.com/apache/logging-log4j-samples/blob/main/log4j-samples-graalvm/src/test/java/org/apache/logging/log4j/samples/graalvm/StandardIT.java) for example).

Start by inspecting the log4j-samples-jlink module and comparing its setup with log4j-samples-graalvm/src/test/java/org/apache/logging/log4j/samples/graalvm/StandardIT.java. Use the Main class as the entry point and make the integration test verify that it works in the custom JLink environment.

Written by the indexing model from the issue text.