CDRIVER-3973: Support the ignoreExtraEvents property (#1012)

This doesn't fully complete the tasks of CDRIVER-3973, but finishes a
single isolated subtask thereof: Supprt for the `ignoreExtraEvents`
property on `expectedEventsForClient`, which is a prereq for enabling
the new changestreams tests.

In this case, if `ignoreExtraEvents` is specified as `true`, we won't
generate an error if the "actual events" contains more events than
"expected events." Each expected event is still searched for within
the "actual events" list, and anything additional is ignored.

Author: vector-of-bool

src/libmongoc/tests/unified/runner.c

@@ -1185,6 +1185,8 @@ test_check_expected_events_for_client (test_t *test,
    bson_parser_t *bp = NULL;
    char *client_id = NULL;
    bson_t *expected_events = NULL;
+   bool just_false = false;
+   bool *ignore_extra_events = &just_false;
    entity_t *entity = NULL;
    bson_iter_t iter;
    event_t *eiter = NULL;
@@ -1195,6 +1197,7 @@ test_check_expected_events_for_client (test_t *test,
    bp = bson_parser_new ();
    bson_parser_utf8 (bp, "client", &client_id);
    bson_parser_array (bp, "events", &expected_events);
+   bson_parser_bool_optional (bp, "ignoreExtraEvents", &ignore_extra_events);
    bson_parser_utf8_optional (bp, "eventType", &event_type);
    if (!bson_parser_parse (bp, expected_events_for_client, error)) {
       goto done;
@@ -1224,11 +1227,19 @@ test_check_expected_events_for_client (test_t *test,
    expected_num_events = bson_count_keys (expected_events);
    LL_COUNT (entity->events, eiter, actual_num_events);
    if (expected_num_events != actual_num_events) {
-      test_set_error (error,
-                      "expected: %" PRIu32 " events but got %" PRIu32,
-                      expected_num_events,
-                      actual_num_events);
-      goto done;
+      bool too_many_events = actual_num_events > expected_num_events;
+      if (*ignore_extra_events) {
+         // We can never have too many events
+         too_many_events = false;
+      }
+      bool too_few_events = actual_num_events < expected_num_events;
+      if (too_few_events || too_many_events) {
+         test_set_error (error,
+                         "expected: %" PRIu32 " events but got %" PRIu32,
+                         expected_num_events,
+                         actual_num_events);
+         goto done;
+      }
    }
 
    eiter = entity->events;