Enable horizontal class merging for all tests

Bug: 163311975
Change-Id: I5d770313f9e1ccd9001f024b0f2679091502fd6f

src/test/java/com/android/tools/r8/TestCompilerBuilder.java

diff --git a/src/test/java/com/android/tools/r8/TestCompilerBuilder.java b/src/test/java/com/android/tools/r8/TestCompilerBuilder.java
index 37f8a22..3d24da4 100644
--- a/src/test/java/com/android/tools/r8/TestCompilerBuilder.java
+++ b/src/test/java/com/android/tools/r8/TestCompilerBuilder.java
@@ -46,6 +46,7 @@
       options -> {
         options.testing.allowClassInlinerGracefulExit = false;
         options.testing.reportUnusedProguardConfigurationRules = true;
+        options.enableHorizontalClassMerging = true;
       };
 
   final Backend backend;