fixing TestTracer

tests/common/src/main/java/org/springframework/cloud/sleuth/test/TestTracer.java

@@ -92,6 +92,11 @@ public class TestTracer implements Tracer, AutoCloseable {
 		return new TestSpanBuilder(delegate.spanBuilder(), this);
 	}
 
+	@Override
+	public TraceContext.Builder traceContextBuilder() {
+		return delegate.traceContextBuilder();
+	}
+
 	@Override
 	@Nullable
 	public SpanCustomizer currentSpanCustomizer() {