cmd: test: pinmux: Do not check all empty spaces

There is really no value to check empty spaces. That's why use
ut_assert_nextlinen() instead of ut_assert_nextline().
This change ensures that PINMUX_SIZE can be increased without changing
tests.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://lore.kernel.org/r/03aecf4c67ec8d72bf2a90baf1516fc5bd300fe0.1646668094.git.michal.simek@xilinx.com