Fix: nextToken value in logs:FilterLogEvents response (#3883)

* Fix: `nextToken` value in `logs:FilterLogEvents` response

Plagiarizing freely from @bpandola and his PR #3398, I have
modified the pagination for FilterLogEvents to more closely follow
the real AWS behaviour.

Fixes #3882

* Black reformatted my code.

* Remove timezone for python2.7 compatibility.

* Hopefully fix python2.7 compatibility for real.

* Additional test for a non-matching log group name in the nextToken.

moto/logs/models.py

@@ -366,13 +366,35 @@ class LogGroup:
         if interleaved:
             events = sorted(events, key=lambda event: event["timestamp"])
 
-        if next_token is None:
-            next_token = 0
+        first_index = 0
+        if next_token:
+            try:
+                group, stream, event_id = next_token.split("/")
+                if group != log_group_name:
+                    raise ValueError()
+                first_index = (
+                    next(
+                        index
+                        for (index, e) in enumerate(events)
+                        if e["logStreamName"] == stream and e["eventId"] == event_id
+                    )
+                    + 1
+                )
+            except (ValueError, StopIteration):
+                first_index = 0
+                # AWS returns an empty list if it receives an invalid token.
+                events = []
 
-        events_page = events[next_token : next_token + limit]
-        next_token += limit
-        if next_token >= len(events):
-            next_token = None
+        last_index = first_index + limit
+        if last_index > len(events):
+            last_index = len(events)
+        events_page = events[first_index:last_index]
+        next_token = None
+        if events_page and last_index < len(events):
+            last_event = events_page[-1]
+            next_token = "{}/{}/{}".format(
+                log_group_name, last_event["logStreamName"], last_event["eventId"]
+            )
 
         searched_streams = [
             {"logStreamName": stream.logStreamName, "searchedCompletely": True}