Use ScopedTempFile in unit test.

Replace these 3 lines of code:

  string path;
  ASSERT_TRUE(utils::MakeTempFile("name-XXXXXX", &path, nullptr));
  ScopedPathUnlinker path_unlinker(path);

with one liner:

  test_utils::ScopedTempFile file("name-XXXXXX");

Bug: None
Test: unit test
Change-Id: Ic5be7dc8339842270023055bcc3a97e526953f04