test: skip tests that XMLSAXParser.warning on non Linux

Because it's not portable.

Author: kou

test/test-xml-sax-parser.lua

@@ -949,6 +949,12 @@ function TestXMLSAXParser.test_warning()
 end
 
 function TestXMLSAXParser.test_set_pedantic_as_option()
+  if jit.os ~= "Linux" then
+    -- parser.warning doesn't work because warningSAXFunc accepts
+    -- variable length arguments.
+    return
+  end
+
   local xml = [[
 <?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE root SYSTEM "file:///usr/local/share/test.dtd" [
@@ -1017,6 +1023,12 @@ local function collect_xml_errors(chunk)
 end
 
 function TestXMLSAXParser.test_xml_structured_error()
+  if jit.os ~= "Linux" then
+    -- parser.warning doesn't work because warningSAXFunc accepts
+    -- variable length arguments.
+    return
+  end
+
   local xml = [[
 <?xml version="1.0"?>
 <id>&aaa;</id>

xmlua/xml-sax-parser.lua

@@ -267,6 +267,8 @@ local function create_end_element_callback(user_callback)
 end
 
 local function create_warning_callback(user_callback)
+  -- This isn't portable because warningSAXFunc accepts variable
+  -- length arguments.
   local callback = function(user_data,
                             raw_warning_message,
                             raw_string_value)