tests: gpiomon: check the whole output string
tests: gpiomon: check the whole output string

Add the 'event:' prefix to the regex pattern when checking the output
of gpiomon.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod