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>

doc/develop/py_testing.rst

@@ -103,6 +103,14 @@ will be written to `${build_dir}/test-log.html`. This is best viewed in a web
 browser, but may be read directly as plain text, perhaps with the aid of the
 `html2text` utility.
 
+If sandbox crashes (e.g. with a segfault) you will see message like this::
+
+
+    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)
+
+
 Controlling output
 ~~~~~~~~~~~~~~~~~~