AU: shift to use new TMPDIR-enabled temp file

The main change here is that delta generator will now create all
temporary files in TMPDIR, if set. Other than that, we're converting all
other temporary file/directory creation to use the new functions.

- All temps of the form "/tmp/foo" are converted to "foo": this
  preserves the behavior in the default case (where TMPDIR is not set),
  yet will do the right thing if run with a different TMPDIR.

- A few other cases (for example, temp file created relative to the
  current working directory) will now be created in TMPDIR or /tmp.
  These are all in unit tests and the transition makes sense anyway.

Note that two temp file/directory creation calls in actual UE code were
using "/tmp/..." and were not changed. This will ensure that they are
resilient to TMPDIR changes and will always be allocated in the same
(hard-coded) location.

BUG=chromium:253622
TEST=Unit tests.

Change-Id: Ia1208963a0e2fcd43b8d6f92bb3d1b7459e930a2
Reviewed-on: https://chromium-review.googlesource.com/182247
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/test_utils.cc b/test_utils.cc
index d00f432..e73146c 100644
--- a/test_utils.cc
+++ b/test_utils.cc
@@ -28,7 +28,7 @@
 
 namespace chromeos_update_engine {
 
-const char* const kMountPathTemplate = "/tmp/UpdateEngineTests_mnt-XXXXXX";
+const char* const kMountPathTemplate = "UpdateEngineTests_mnt-XXXXXX";
 
 bool WriteFileVector(const std::string& path, const std::vector<char>& data) {
   return utils::WriteFile(path.c_str(), &data[0], data.size());
@@ -300,7 +300,7 @@
 ScopedLoopMounter::ScopedLoopMounter(const string& file_path,
                                      string* mnt_path,
                                      unsigned long flags) {
-  EXPECT_TRUE(utils::MakeTempDirectory("/tmp/mnt.XXXXXX", mnt_path));
+  EXPECT_TRUE(utils::MakeTempDirectory("mnt.XXXXXX", mnt_path));
   dir_remover_.reset(new ScopedDirRemover(*mnt_path));
 
   string loop_dev;