test_helpers: Fix POSTRequestMock typing.

The proximal issue here is that in upcoming commits, we're going to
change the type of the `anchor` field in `get_messages_backend` to
support passing either an integer or a string.

Many of our tests using POSTRequestMock currently define a query
object that uses integer values for the integer fields we're going to
pass into it, e.g. {'num_after': 0}.  That is the correct type for
that field in the Zulip API, before HTTP encoding turns it into a
string.  However, because POSTRequestMock didn't use HTTP encoding at
all (which will convert the 0 into a '0'), it ended up passing an
integer to a function that can't possible receive one as an argument.

Ideally, we'd just get rid of POSTRequestMock, since it's a hack, and
just do real HTTP requests instead.

But since it's used in a lot of places making doing so somewhat
impractical, we can get past this issue by just making POSTRequestMock
convert integers to strings.

zerver/lib/test_helpers.py

@@ -256,7 +256,15 @@ class POSTRequestMock:
 
     def __init__(self, post_data: Dict[str, Any], user_profile: Optional[UserProfile]) -> None:
         self.GET = {}  # type: Dict[str, Any]
-        self.POST = post_data
+
+        # Convert any integer parameters passed into strings, even
+        # though of course the HTTP API would do so.  Ideally, we'd
+        # get rid of this abstraction entirely and just use the HTTP
+        # API directly, but while it exists, we need this code.
+        self.POST = {}  # type: Dict[str, str]
+        for key in post_data:
+            self.POST[key] = str(post_data[key])
+
         self.user = user_profile
         self._tornado_handler = DummyHandler()
         self._log_data = {}  # type: Dict[str, Any]