Enable class merging for class merging tests.

BUG=

Change-Id: Idfe1b328e39ac1222fdabe2423670a8d6479ccd6
diff --git a/src/test/java/com/android/tools/r8/classmerging/ClassMergingTest.java b/src/test/java/com/android/tools/r8/classmerging/ClassMergingTest.java
index 2326eb5..0d0d6f0 100644
--- a/src/test/java/com/android/tools/r8/classmerging/ClassMergingTest.java
+++ b/src/test/java/com/android/tools/r8/classmerging/ClassMergingTest.java
@@ -35,7 +35,10 @@
             .addProgramFiles(EXAMPLE_JAR)
             .addProguardConfigurationFiles(EXAMPLE_KEEP)
             .setMinification(false)
-            .build(), o -> o.allowAccessModification = false);
+            .build(), o -> {
+          o.allowAccessModification = false;
+          o.skipClassMerging = false;
+        });
     inspector = new DexInspector(
         Paths.get(temp.getRoot().getCanonicalPath()).resolve("classes.dex"));
   }