Add utility to testbase for testing on current runtime

This is helpful for generating only a single test that will run a
reference test without using R8.

Change-Id: I40c49c6d8a2e78b7bffb261dd393d2abc43dedcf
diff --git a/src/test/java/com/android/tools/r8/TestBase.java b/src/test/java/com/android/tools/r8/TestBase.java
index 9b8abe6..797d185 100644
--- a/src/test/java/com/android/tools/r8/TestBase.java
+++ b/src/test/java/com/android/tools/r8/TestBase.java
@@ -147,6 +147,17 @@
     return testForJvm(temp);
   }
 
+  public TestBuilder<?, ?> testForRuntime(TestRuntime runtime, AndroidApiLevel apiLevel) {
+    if (runtime.isCf()) {
+      return testForJvm();
+    } else {
+      assert runtime.isDex();
+      D8TestBuilder d8TestBuilder = testForD8();
+      d8TestBuilder.setMinApi(apiLevel);
+      return d8TestBuilder;
+    }
+  }
+
   public ProguardTestBuilder testForProguard() {
     return testForProguard(temp);
   }