base: allow creating tempfile in a custom temporary dir.

Bug: http://b/70232908
Test: none.
Change-Id: I24b894793f6725186b3582a91db083427e51aa5a
diff --git a/base/test_utils.cpp b/base/test_utils.cpp
index 1cfa9e6..9d8dfb2 100644
--- a/base/test_utils.cpp
+++ b/base/test_utils.cpp
@@ -84,6 +84,10 @@
   init(GetSystemTempDir());
 }
 
+TemporaryFile::TemporaryFile(const std::string& tmp_dir) {
+  init(tmp_dir);
+}
+
 TemporaryFile::~TemporaryFile() {
   if (fd != -1) {
     close(fd);