pytest: Show a message when sandbox crashes

When a test hands on a real board there is no way on the console to obtain
any information about why it hung.

With sandbox we can actually find out that it died and get a signal or
exit code. Add this to make it easier to figure out what happened.

So instead of:

test/py/u_boot_spawn.py:171: in expect
    c = os.read(self.fd, 1024).decode(errors='replace')
E   OSError: [Errno 5] Input/output error

We get:

test/py/u_boot_spawn.py:171: in expect
    c = os.read(self.fd, 1024).decode(errors='replace')
E   ValueError: U-Boot exited with signal 11 (Signals.SIGSEGV)

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