Migrate dd-trace-core groovy files to java part 5

we migrate 4 tests:
 - DDSpanContextTest
 - DDSpanSerializationTest
 - DDSpanTest
 - KnuthSamplingRateTest

# Conflicts:
#	dd-trace-core/src/test/groovy/datadog/trace/core/DDSpanContextTest.groovy
#	dd-trace-core/src/test/groovy/datadog/trace/core/DDSpanTest.groovy
#	dd-trace-core/src/test/groovy/datadog/trace/core/KnuthSamplingRateTest.groovy

Author: jpbempel

dd-trace-core/src/main/java/datadog/trace/common/writer/ddagent/TraceMapperV0_5.java

@@ -125,6 +125,16 @@ public String endpoint() {
     return "v0.5";
   }
 
+  // Visible for tests
+  Map<Object, Integer> getEncoding() {
+    return encoding;
+  }
+
+  // Visible for tests
+  GrowableBuffer getDictionary() {
+    return dictionary;
+  }
+
   private static class DictionaryMapper implements Mapper<Object> {
 
     @Override

dd-trace-core/src/main/java/datadog/trace/core/PendingTrace.java

@@ -233,6 +233,11 @@ void setLongRunningTrackedState(int state) {
     LONG_RUNNING_STATE.set(this, state);
   }
 
+  // @VisibleForTesting
+  int getPendingReferenceCount() {
+    return pendingReferenceCount;
+  }
+
   boolean empty() {
     return 0 >= COMPLETED_SPAN_COUNT.get(this) + PENDING_REFERENCE_COUNT.get(this);
   }