crash_reporter: Fix unit tests to use ScopedTempDir

Some of the unit tests manually create their own temp
directories from a relative path.  This creates a temp folder
in the folder that the test is run from.  This is a problem
when using test_droid or a non-interactive ADB shell command,
since the root directory is not writable.  Instead use a
ScopedTempDir to handle temporary folders.

Bug: 26671657
Change-Id: Iaf71d345565dcb4393e80426784a864f7f8b5929
Test: Unit tests pass when running brillo_WhitelistedGtests