message: fix session test
  After libmongoc no longer pools unused sessions.
  Fixes a test that was relying on this behavior to use the session it is
  expecting to pool.
diff: @@ -129,6 +129,9 @@ TEST_CASE("session", "[session]") {
  auto a_id = value(session_a->id());
  auto b_id = value(session_b->id());
  + c["db"].run_command(*session_a, make_document(kvp("ping", 1)));
  + c["db"].run_command(*session_b, make_document(kvp("ping", 1)));
  +
  // End session A, then session B.
  session_a = nullptr;
  session_b = nullptr;
  