tests: put temporary file outside the working directory for test portability test-largefiles-update.t creates temporary file exec-bit.patch inside the working directory for no-execbit platform specific test, but subsequent tests aren't aware of it. On execbit platform, subsequent tests can run successfully, because exec-bit.patch isn't created. But on no-execbit platform, this temporary file makes subsequent tests show "? exec-bit.patch" at each "hg status".