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/utils_unittest.cc b/utils_unittest.cc
index c1f9a2e..c947c0b 100644
--- a/utils_unittest.cc
+++ b/utils_unittest.cc
@@ -182,7 +182,7 @@
 
 TEST(UtilsTest, IsSymlinkTest) {
   string temp_dir;
-  EXPECT_TRUE(utils::MakeTempDirectory("/tmp/symlink-test.XXXXXX", &temp_dir));
+  EXPECT_TRUE(utils::MakeTempDirectory("symlink-test.XXXXXX", &temp_dir));
   string temp_file = temp_dir + "temp-file";
   EXPECT_TRUE(utils::WriteFile(temp_file.c_str(), "", 0));
   string temp_symlink = temp_dir + "temp-symlink";
@@ -275,7 +275,7 @@
 
 TEST(UtilsTest, RunAsRootGetFilesystemSizeTest) {
   string img;
-  EXPECT_TRUE(utils::MakeTempFile("/tmp/img.XXXXXX", &img, NULL));
+  EXPECT_TRUE(utils::MakeTempFile("img.XXXXXX", &img, NULL));
   ScopedPathUnlinker img_unlinker(img);
   CreateExtImageAtPath(img, NULL);
   // Extend the "partition" holding the file system from 10MiB to 20MiB.