Change tests to use a temporary dir for files.

Avoid using hard-coded paths for generated files. This can cause problems
if the tests are run in parallel.

Also fix a potential race condition in the unwind_through_unreadable_elf_remote
test.

Test: Ran tests in parallel and normally.
Change-Id: Ib42522de49499766a29bda5bfffe737b56715e3f