Add eol to listchars for mouse specification tests

This is consistent with VIM behavior. When showing a visual selection,
VIM only extends it past the last character if eol is in listchars (even
if nolist is set).

test/functional/ui/mouse_spec.lua

@@ -14,6 +14,7 @@ describe('Mouse input', function()
   before_each(function()
     clear()
     nvim('set_option', 'mouse', 'a')
+    nvim('set_option', 'listchars', 'eol:$')
     -- set mouset to very high value to ensure that even in valgrind/travis,
     -- nvim will still pick multiple clicks
     nvim('set_option', 'mouset', 5000)