file write serialization tests

[benibela]

There are no tests for file:write in https://github.com/expath/expath-cg/blob/master/tests/qt3/file/file.xml

All the serialization tests from qt3 could be imported for it

One case, I am not sure about, is the expected output of file:write("output.json", "{""a"": ""foo""}" ). Is it {"a": "foo"} or {"a": "foo"}. Probably former, but that is useless..