test: dont os.exit() in session disconnect trigger

We are going to make Tarantool shutdown graceful. Particularly for
iproto graceful shutdown we wait while all connections are finished.
On connection disconnect session disconnect triggers are invoked. If
trigger hangs then graceful shutdown will hang too. And os.exit() do
hang, it sleeps forever.

So 110-disconnect-trigger-check.t test now hangs with iproto graceful
shutdown patch.

Making os.exit() from disconnect trigger does not seem as a product
case. So let's just change the test not to use os.exit().

Required for full-ci pass of tarantool/tarantool#9457

t/110-disconnect-trigger-check.t

@@ -16,7 +16,7 @@ test:plan(2)
 -- Verify that _queue_taken space is empty.
 local function check_result()
     if tube == nil then
-        os.exit(1)
+        return
     end
 
     -- tube:drop() is most simple way to check that _queue_taken
@@ -26,7 +26,6 @@ local function check_result()
     test:is(res, true, 'tube:drop() result is true')
 
     tnt.finish()
-    os.exit(test:check() and 0 or 1)
 end
 
 -- Yield in queue's on_disconnect trigger (which handles a client
@@ -92,7 +91,7 @@ local function test_lost_session_id_after_yield()
     -- session' error in the _on_consumer_disconnect and so the
     -- second on_disconnect trigger (check_result) will not be
     -- fired.
-    os.exit(1)
+    os.exit(test:check() and 0 or 1)
 end
 
 test_lost_session_id_after_yield()