base: allow creating tempfile in a custom temporary dir.

Bug: http://b/70232908
Test: none.
Change-Id: I24b894793f6725186b3582a91db083427e51aa5a
diff --git a/include/android-base/test_utils.h b/include/android-base/test_utils.h
index 07a5edd..4cfa06b 100644
--- a/include/android-base/test_utils.h
+++ b/include/android-base/test_utils.h
@@ -24,6 +24,7 @@
 class TemporaryFile {
  public:
   TemporaryFile();
+  explicit TemporaryFile(const std::string& tmp_dir);
   ~TemporaryFile();
 
   // Release the ownership of fd, caller is reponsible for closing the