test/py: Rework how test_ums.py handles (not) having write enabled

test/py: Rework how test_ums.py handles (not) having write enabled

With a newer pylint version we get a warning about how mounted_test_fn
could be used before assignment. Evaluating the code, this can't happen
because we check for "not have_writable_fs_partition" and return before
moving to the part of the tests which use mounted_test_fn. However, we
should instead have this written so that we only try this part of the
test if have_writable_fs_partition is set, and this also fixes the
warning. As part of this we also move test_f and mounted_test_fn to the
section of code that already only does this if
have_writable_fs_partition is set.

Signed-off-by: Tom Rini <trini@konsulko.com>