tests/functional: Speed up the avr_mega2560 test

tests/functional: Speed up the avr_mega2560 test

We can simply check for the expected pattern on the console,
no need to wait for two seconds here to search for the pattern
in the log at the end.

While we're at it, also remove the obsolete "timeout" variable
from this test.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250603184710.25651-1-thuth@redhat.com>