Update ProGuard config for MethodHandleTestRunner

The test implicitly assumes that MethodHandleTest$I is not merged into MethodHandleTest$Impl (due to the use of reflection).

Change-Id: Iae8d603bfd1227cbc334c9628eabd3a0b30dd3d2
diff --git a/src/test/java/com/android/tools/r8/cf/MethodHandleTestRunner.java b/src/test/java/com/android/tools/r8/cf/MethodHandleTestRunner.java
index 6069b4c..da32cce 100644
--- a/src/test/java/com/android/tools/r8/cf/MethodHandleTestRunner.java
+++ b/src/test/java/com/android/tools/r8/cf/MethodHandleTestRunner.java
@@ -31,7 +31,6 @@
 import org.junit.runners.Parameterized;
 import org.junit.runners.Parameterized.Parameters;
 
-
 @RunWith(Parameterized.class)
 public class MethodHandleTestRunner extends TestBase {
   static final Class<?> CLASS = MethodHandleTest.class;
@@ -169,7 +168,9 @@
           Arrays.asList(
               "-keep public class com.android.tools.r8.cf.MethodHandleTest {",
               "  public static void main(...);",
-              "}"),
+              "}",
+              // Disallow merging MethodHandleTest$I into MethodHandleTest$Impl
+              "-keep public interface com.android.tools.r8.cf.MethodHandleTest$I"),
           Origin.unknown());
     }
     try {