Test: Relax jarhell gradle test (#33787)

Gradle can sometimes emit mixed log lines due to how we spawn things in
separate processes. This commit changes the jarhell integ test to only
look for the exception and message, instead of including the outer part
about the exception in thread main.

closes #33774

Author: rjernst

buildSrc/src/test/java/org/elasticsearch/gradle/precommit/JarHellTaskIT.java

@@ -34,7 +34,7 @@ public void testJarHellDetected() {
         assertTaskFailed(result, ":jarHell");
         assertOutputContains(
             result.getOutput(),
-            "Exception in thread \"main\" java.lang.IllegalStateException: jar hell!",
+            "java.lang.IllegalStateException: jar hell!",
             "class: org.apache.logging.log4j.Logger"
         );
     }