test: Avoid double echo when starting up

test: Avoid double echo when starting up

There is a very annoying bug at present where the terminal echos part
of the first command sent to the board. This happens because the
terminal is still set to echo for a period until Labgrid starts up and
can change this.

Fix this by disabling echo (and other terminal features) as soon as the
spawn happens.

Signed-off-by: Simon Glass <sjg@chromium.org>