tests/test_uffd: revert to correct working dir
tests/test_uffd: revert to correct working dir

`spawn_pf_handler` changes the current working directory to `/`
which causes some tests that are run after this call to fail.
Revert to the correct working directory before exiting from
spawn_pf_handler.

Signed-off-by: Alexandru-Cezar Sardan <490cf4494272cbda36b12d85d8b4db493ab05419@amazon.com>

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