Add a test for #186

Was hard to reproduce, doesn't happen with many other layouts

tests/tst_docks.h

@@ -240,6 +240,7 @@ private Q_SLOTS:
     void tst_redocksToPreviousTabIndex();
     void tst_toggleTabbed();
     void tst_toggleTabbed2();
+    void tst_resizePropagatesEvenly();
 
     void tst_addMDIDockWidget();
     void tst_redockToMDIRestoresPosition();