Pexpect: Return the match object instead of the result

The result is just the *index* of the pattern that matched. But since
we never pass a *list* it's just always 0.

spawn.match is the MatchObject that produced the match, so it can be
used to post-process the matched output, e.g.

```python
m = expect_re('\d+')
m.group() # is now the matched number
```

build_tools/pexpect_helper.py

@@ -182,12 +182,15 @@ class SpawnedProc(object):
             On failure, this prints an error and exits.
         """
         try:
-            res = self.spawn.expect(pat, **kwargs)
+            self.spawn.expect(pat, **kwargs)
             when = self.time_since_first_message()
             self.messages.append(
                 Message.received_output(self.spawn.match.group(), when)
             )
-            return res
+            # When a match is found,
+            # spawn.match is the MatchObject that produced it.
+            # This can be used to check what exactly was matched.
+            return self.spawn.match
         except pexpect.ExceptionPexpect as err:
             if not pat_desc:
                 pat_desc = str(pat)