Fix failing logging audit tests

This commit fixes the logging audit tests which were broken due to an
upstream change in core Elasticsearch relating to the fact that prefixes
are no longer considered part of the log message, but are instead
implemented via markers.

Original commit: elastic/x-pack-elasticsearch@abd7ec23d8

elasticsearch/x-pack/security/src/test/java/org/elasticsearch/xpack/security/audit/logfile/CapturingLogger.java

@@ -57,7 +57,7 @@ public class CapturingLogger {
         public final List<String> trace = new ArrayList<>();
 
         private MockAppender(final String name) throws IllegalAccessException {
-            super(name, RegexFilter.createFilter(".*(\n.*)*", new String[0], true, null, null), null);
+            super(name, RegexFilter.createFilter(".*(\n.*)*", new String[0], false, null, null), null);
         }
 
         @Override