test/functional: clean up according to luacheck (part 1)

test/functional/shell/viml_system_spec.lua

@@ -133,7 +133,7 @@ describe('system()', function()
       -- write more than 1mb of data, which should be enough to overcome
       -- the os buffer limit and force multiple event loop iterations to write
       -- everything
-      for i = 1, 0xffff do
+      for _ = 1, 0xffff do
         input[#input + 1] = '01234567890ABCDEFabcdef'
       end
       input = table.concat(input, '\n')
@@ -299,7 +299,7 @@ describe('systemlist()', function()
   describe('passing a lot of input', function()
     it('returns the program output', function()
       local input = {}
-      for i = 1, 0xffff do
+      for _ = 1, 0xffff do
         input[#input + 1] = '01234567890ABCDEFabcdef'
       end
       nvim('set_var', 'input', input)