test/unit: clean up according to luacheck

test/unit/os/shell_spec.lua

@@ -17,7 +17,7 @@ local shell = helpers.cimport(
   './src/nvim/main.h',
   './src/nvim/misc1.h'
 )
-local ffi, eq, neq = helpers.ffi, helpers.eq, helpers.neq
+local ffi, eq = helpers.ffi, helpers.eq
 local intern = helpers.internalize
 local to_cstr = helpers.to_cstr
 local NULL = ffi.cast('void *', 0)
@@ -70,7 +70,7 @@ describe('shell functions', function()
     end)
 
     it ('returns non-zero exit code', function()
-      local status, output = os_system('exit 2')
+      local status = os_system('exit 2')
       eq(2, status)
     end)
   end)