Add synchronized to output consumer callback in tests

The lack of a lock can result in writing to the output collection on
multiple threads and the test will then fail.

Change-Id: I71ac2b5bf7c800194220488fe70224e539160653
diff --git a/src/test/java/com/android/tools/r8/d8/IntermediateModeMarkerTest.java b/src/test/java/com/android/tools/r8/d8/IntermediateModeMarkerTest.java
index 5062525..ab92812 100644
--- a/src/test/java/com/android/tools/r8/d8/IntermediateModeMarkerTest.java
+++ b/src/test/java/com/android/tools/r8/d8/IntermediateModeMarkerTest.java
@@ -61,7 +61,7 @@
         .setProgramConsumer(
             new DexFilePerClassFileConsumer.ForwardingConsumer(null) {
               @Override
-              public void accept(
+              public synchronized void accept(
                   String primaryClassDescriptor,
                   ByteDataView data,
                   Set<String> descriptors,
@@ -83,7 +83,7 @@
         .setProgramConsumer(
             new DexFilePerClassFileConsumer.ForwardingConsumer(null) {
               @Override
-              public void accept(
+              public synchronized void accept(
                   String primaryClassDescriptor,
                   ByteDataView data,
                   Set<String> descriptors,
@@ -106,7 +106,7 @@
         .setProgramConsumer(
             new DexIndexedConsumer.ForwardingConsumer(null) {
               @Override
-              public void accept(
+              public synchronized void accept(
                   int fileIndex,
                   ByteDataView data,
                   Set<String> descriptors,