twister: treat imgtool image exceeded as flash overflow

Twister detects FLASH overflow and skips tests that trigger that
condition by default, but sometimes images are just on the flash size
limit and then overflows at a later stage when imgtool adds a trailer,
which gets detected as a build fail and fails the run.

This detects the imgtool flash overflow together with the normal build
ones, causing the build to be skipped on imgtool flash overflow as well.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>