Modify XML reference files only on "--update"
Modify XML reference files only on "--update"

Commit ef71e75 introduced a change whereby run_functional_tests.lua
will always rewrite the .xml files whenever check_xml_output() is
executed.

I think it's preferable to modify the files only if an actual
"--update" was explicitly requested. This patch changes the code
accordingly.

GuntherStruyf/luaunit,GuntherStruyf/luaunit