Update xml, fix + improve robustness of unions.lua unit tests

- unit_action_fields(): handled primitive union members correctly
- unit_action_type(): added messages to make failures easier to diagnose
    - Also removed redundant checks that effectively checked that
      `enum.attrs[k] == enum.attrs[v]` - this is out of scope of union tests

library/xml

@@ -1 +1 @@
-Subproject commit ad1c98cf852936694300eaf8d69e91c10b7ea57f
+Subproject commit 9a936001d9095681d5cb6225eb18bfe0ed2bcf28

test/structures/unions.lua

@@ -3,7 +3,7 @@ local utils = require('utils')
 function test.unit_action_fields()
     dfhack.with_temp_object(df.unit_action:new(), function(action)
         for k in pairs(action.data) do
-            expect.eq(utils.addressof(action.data.raw_data), utils.addressof(action.data[k]),
+            expect.eq(utils.addressof(action.data.raw_data), utils.addressof(action.data:_field(k)),
                 'address of ' .. k .. ' does not match')
         end
     end)
@@ -11,9 +11,12 @@ end
 
 function test.unit_action_type()
     dfhack.with_temp_object(df.unit_action:new(), function(action)
-        for k, v in ipairs(df.unit_action_type) do
-            expect.true_(action.data[df.unit_action_type.attrs[k].tag])
-            expect.true_(action.data[df.unit_action_type.attrs[v].tag])
+        for index, name in ipairs(df.unit_action_type) do
+            expect.true_(name, "unit_action_type entry without name: " .. tostring(index))
+            local tag = df.unit_action_type.attrs[name].tag
+            expect.true_(tag, "unit_action_type entry missing tag: name=" .. name)
+            expect.pairs_contains(action.data, tag,
+                "unit_action_type entry missing from unit_action.data: name=" .. name)
         end
     end)
 end