test/py: test pinmux command

Add pinmux test which test the following commands:
  - pinmux list
  - pinmux dev
  - pinmux status

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Mark some tests as sandbox-centric]
Signed-off-by: Tom Rini <trini@konsulko.com>

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