Fix UnsupportedOperationException (eclipse-tycho#2938)

OfflineModeTest fails with UnsupportedOperationException as HttpServer
no longer allows direct access to access logs. Change OfflineModeTest to
dedicated and limited mutation API for access logs.

tycho-its/src/test/java/org/eclipse/tycho/test/target/OfflineModeTest.java

@@ -92,7 +92,7 @@ private void runAndVerifyOnlineBuild(Verifier verifier) throws VerificationExcep
 		verifier.executeGoal("integration-test");
 		verifier.verifyErrorFreeLog();
 		assertFalse(server.getAccessedUrls("test").isEmpty());
-		server.getAccessedUrls("test").clear();
+		server.clear();
 	}
 
 }

tycho-its/src/test/java/org/eclipse/tycho/test/util/HttpServer.java

@@ -203,6 +203,17 @@ public List<String> getAccessedUrls(String contextName) {
 		}
 	}
 
+	/**
+	 * Reset state.
+	 * 
+	 * Clear access logs. Does not affect configuration.
+	 **/
+	public void clear() {
+		synchronized (contextName2accessedUrls) {
+			contextName2accessedUrls.clear();
+		}
+	}
+
 	private void registerContext(ServletContextHandler context) {
 		context.addFilter(new FilterHolder(new Monitoring()), "*", EnumSet.of(DispatcherType.REQUEST));
 		contexts.addHandler(context);