lib/mnit: introduce `mnit_injected_input` to inject events into mnit applications

Signed-off-by: Jean Privat <jean@pryen.org>
lib/mnit: introduce `mnit_null` to execute mnit without a display

Signed-off-by: Jean Privat <jean@pryen.org>
shoot: add a `null` target that uses mnit::mnit_null

Signed-off-by: Jean Privat <jean@pryen.org>
tests: teach tests.sh to use MNIT_* environment variables

Signed-off-by: Jean Privat <jean@pryen.org>
tests_full: add mnit_null variants, and the .inputs file for shoot

Signed-off-by: Jean Privat <jean@pryen.org>