JNI test improvements.

I broke some tests with a last-minute alteration to my previous change,
and I missed a good suggestion for an extra check.

Change-Id: I9dd19aab033bcec6bc61b35c46b5c740869c373b
diff --git a/src/common_test.h b/src/common_test.h
index 54c9bb7..52c3393 100644
--- a/src/common_test.h
+++ b/src/common_test.h
@@ -542,6 +542,7 @@
   ~CheckJniAbortCatcher() {
     vm_->check_jni_abort_hook = NULL;
     vm_->check_jni_abort_hook_data = NULL;
+    EXPECT_TRUE(actual_.empty()) << actual_;
   }
 
   void Check(const char* expected_text) {