ART: Fix assembler_test to use ScratchFile

This removes a warning about tmpnam usage.

Also add an assertion to ScratchFile about ANDROID_DATA being set,
which it relies on for the temp directory.

Change-Id: I1202f92e48e61492f0ed3ac36ff44fde34dbb0e1
diff --git a/runtime/common_runtime_test.h b/runtime/common_runtime_test.h
index 723e32c..79d3690 100644
--- a/runtime/common_runtime_test.h
+++ b/runtime/common_runtime_test.h
@@ -57,6 +57,9 @@
 class ScratchFile {
  public:
   ScratchFile() {
+    // ANDROID_DATA needs to be set
+    CHECK_NE(static_cast<char*>(nullptr), getenv("ANDROID_DATA")) <<
+        "Are you subclassing RuntimeTest?";
     filename_ = getenv("ANDROID_DATA");
     filename_ += "/TmpFile-XXXXXX";
     int fd = mkstemp(&filename_[0]);