Tweaks around accessing SuiteResult

Closes testng-team#3078

Following changes were made:

* Removed the lock based synchronization around 
SuiteResult because it’s already backed by a 
SynchronizedMap from Collections.
* Altered the getter such that it returns back a
regular linkedHashMap (without the synchronisation
Because users are expected ONLY to iterate on it and
NOT change its state)
* Also just to ensure that users don’t garble the 
Suite result, wrapping it with an UnModifiableMap

testng-core/src/main/java/org/testng/SuiteRunner.java

@@ -415,16 +415,12 @@ private void runSequentially() {
     }
   }
 
-  private final AutoCloseableLock suiteResultsLock = new AutoCloseableLock();
-
   private void runTest(TestRunner tr) {
     visualisers.forEach(tr::addListener);
     tr.run();
 
     ISuiteResult sr = new SuiteResult(xmlSuite, tr);
-    try (AutoCloseableLock ignore = suiteResultsLock.lock()) {
-      suiteResults.put(tr.getName(), sr);
-    }
+    suiteResults.put(tr.getName(), sr);
   }
 
   /**
@@ -514,7 +510,12 @@ public String getOutputDirectory() {
 
   @Override
   public Map<String, ISuiteResult> getResults() {
-    return suiteResults;
+    // When users want to access the results, they are NOT expected to alter it's state.
+    // So we don't need to be sending back a synchronized map which adds a small additional cost
+    // to enter and exit the synchronized block.
+    // Also just to ensure that we guard the internals of the suite results we now wrap it
+    // around with an unmodifiable map.
+    return Collections.unmodifiableMap(new LinkedHashMap<>(suiteResults));
   }
 
   /**