Fix test broken by the TMPDIR change

The change in 6ea3e39a100ca3ca2f8c36ea30e6c8e60a50ef70 to make TMPDIR
point to the work directory of the test case broke the
run_test_case__atf__required_user__atf__unprivileged__atf__drop test
in engine/test_case_test.

The breakage came because the unprivileged test case spawned by this
test case had its work directory within the work directory of the
container test case, which in turn had its work directory set to
root with 0700 permissions.  This prevented the unprivileged test
case from creating its results file as absolute paths could not be
resolved.

Fix this by weakening the permissions of the first work directory.