test: use exec_capture() in more places (#22787)

Problem:
Using `meths.exec2("code", { output = true })` is too verbose.

Solution:
Use exec_capture() in more places.

runtime/lua/vim/_editor.lua

@@ -338,7 +338,8 @@ vim.cmd = setmetatable({}, {
     if type(command) == 'table' then
       return vim.api.nvim_cmd(command, {})
     else
-      return vim.api.nvim_exec2(command, { output = false }).output
+      vim.api.nvim_exec2(command, {})
+      return ''
     end
   end,
   __index = function(t, command)