test/terminal: fix indeterminism in colorcolumn screen test

Previously, the screen test was expecting the screen state to be
identical to the previous screen test in `thelpers.screen_setup()`,
which is indeterministic. (The later screen test can accidentally
still see the previous identical state). The solution is to add a test
for a intermediate different state.

test/functional/terminal/window_spec.lua

@@ -14,8 +14,17 @@ describe('terminal window', function()
 
   describe('with colorcolumn set', function()
     before_each(function()
-      feed('<c-\\><c-n>:set colorcolumn=20<cr>i')
-      wait()
+      feed('<c-\\><c-n>')
+      screen:expect([[
+        tty ready                                         |
+        {2: }                                                 |
+                                                          |
+                                                          |
+                                                          |
+        ^                                                  |
+                                                          |
+      ]])
+      feed(':set colorcolumn=20<cr>i')
     end)
 
     it('wont show the color column', function()