Fix testTracerLog Network Tests

* Start appender before using it like we do for e.g. the Netty leak detection appender to avoid interference from actions on the network threads that might still be dangling from previous tests in the same suite
* Closes elastic#41890

Author: original-brownbear

test/framework/src/main/java/org/elasticsearch/transport/AbstractSimpleTransportTestCase.java

@@ -1047,10 +1047,9 @@ public String executor() {
             .build());
 
         MockLogAppender appender = new MockLogAppender();
+        appender.start();
         Loggers.addAppender(LogManager.getLogger("org.elasticsearch.transport.TransportService.tracer"), appender);
         try {
-            appender.start();
-
             final String requestSent = ".*\\[internal:test].*sent to.*\\{TS_B}.*";
             final MockLogAppender.LoggingExpectation requestSentExpectation =
                 new MockLogAppender.PatternSeenEventExpectation(