Add precise stds for Lua libs

TODO: more prexise definition of ngx_lua and busted stds
TODO: change _G std to equal to std of currently used Lua version

spec/cli_spec.lua

@@ -584,7 +584,7 @@ Total: 1 warning / 0 errors in 1 file
 Checking spec/samples/read_globals.lua            5 warnings
 
     spec/samples/read_globals.lua:1:1: setting read-only global variable 'string'
-    spec/samples/read_globals.lua:2:1: setting read-only field 'append' of global 'table'
+    spec/samples/read_globals.lua:2:1: setting undefined field 'append' of global 'table'
     spec/samples/read_globals.lua:5:1: setting read-only global variable 'bar'
     spec/samples/read_globals.lua:6:1: mutating non-standard global variable 'baz'
     spec/samples/read_globals.lua:6:21: accessing undefined variable 'baz'
@@ -598,7 +598,7 @@ Total: 5 warnings / 0 errors in 1 file
 Checking spec/samples/indirect_globals.lua        3 warnings
 
     spec/samples/indirect_globals.lua:2:11-16: accessing undefined variable 'global'
-    spec/samples/indirect_globals.lua:5:1-8: indirectly setting read-only field 'concat.foo.bar' of global 'table'
+    spec/samples/indirect_globals.lua:5:1-8: indirectly setting undefined field 'concat.foo.bar' of global 'table'
     spec/samples/indirect_globals.lua:5:32-37: accessing undefined variable 'global'
 
 Total: 3 warnings / 0 errors in 1 file
@@ -621,7 +621,7 @@ Total: 2 warnings / 0 errors in 1 file
 Checking spec/samples/read_globals.lua            5 warnings
 
     spec/samples/read_globals.lua:1:1: (W121) setting read-only global variable 'string'
-    spec/samples/read_globals.lua:2:1: (W122) setting read-only field 'append' of global 'table'
+    spec/samples/read_globals.lua:2:1: (W142) setting undefined field 'append' of global 'table'
     spec/samples/read_globals.lua:5:1: (W121) setting read-only global variable 'bar'
     spec/samples/read_globals.lua:6:1: (W112) mutating non-standard global variable 'baz'
     spec/samples/read_globals.lua:6:21: (W113) accessing undefined variable 'baz'

spec/options_spec.lua

@@ -141,7 +141,9 @@ describe("options", function()
          assert.is_same({read_only = false, other_fields = true}, std.fields.bar)
          assert.is_nil(std.fields.baz)
          assert.is_same({read_only = true, deep_read_only = true, other_fields = true}, std.fields.quux)
-         assert.is_same({other_fields = true, deep_read_only = true}, std.fields.string)
+         assert.is_table(std.fields.string)
+         assert.is_true(std.fields.string.deep_read_only)
+         assert.is_nil(std.fields.string.other_fields)
       end)
 
       it("considers read-only and regular field definitions", function()