Have workcache::test put `foo.c` in the same directory it runs in.

This prevents a stray `foo.c` from being left lying around after tests
run, and it's more consistent with the rest of the code.
auto merge of #9204 : lkuper/rust/workcache, r=alexcrichton

This just removes the stray "foo.c" that `workcache::test` leaves behind when, for example, `make check` runs.