Assert that the specific android.jar version exists in tests.

R=herhut

Change-Id: I708d4d7571418c9ec49e83f422a579581cb72643
diff --git a/src/test/java/com/android/tools/r8/ToolHelper.java b/src/test/java/com/android/tools/r8/ToolHelper.java
index d3e9d72..ef0221d 100644
--- a/src/test/java/com/android/tools/r8/ToolHelper.java
+++ b/src/test/java/com/android/tools/r8/ToolHelper.java
@@ -480,9 +480,12 @@
       // TODO(mikaelpeltier) Android P does not yet have his android.jar use the O version
       minSdkVersion = AndroidApiLevel.O.getLevel();
     }
-    return String.format(
+    String jar = String.format(
         ANDROID_JAR_PATTERN,
         minSdkVersion == AndroidApiLevel.getDefault().getLevel() ? DEFAULT_MIN_SDK : minSdkVersion);
+    assert Files.exists(Paths.get(jar))
+        : "Expected android jar to exist for API level " + minSdkVersion;
+    return jar;
   }
 
   public static Path getJdwpTestsCfJarPath(int minSdk) {