test(diff): adjust to Nightly change in how extmark is invalidated

Now on Nightly after extmark with sign data is invalidated, signs are
immediately cleaned leading to possibly hiding the whole sign column
with the default 'signcolumn=auto'. Force 'signcolumn=yes' instead.

tests/test_diff.lua

@@ -2013,6 +2013,7 @@ end
 
 T['Visualization']['reacts to hunk lines delete/move'] = function()
   if child.fn.has('nvim-0.10') == 0 then MiniTest.skip('Reaction to line delete/move is available on Neovim>0.10.') end
+  child.o.signcolumn = 'yes'
 
   set_lines({ 'aaa', 'bbb', 'uuu', 'vvv', 'ccc', 'ddd' })
   set_ref_text(0, { 'aaa', 'bbb', 'ccc', 'ddd' })