json: replace TRef class by TNamed class in testing

[linev]

TRef has custom streamer and not correctly supported by JSON I/O

[phsft-bot]

Starting build on centos7/gcc49, mac1013/native, slc6/gcc49, slc6/gcc62, slc6/gcc62, ubuntu16/native, ubuntu16/native, windows10/vc15 with flags -Dvc=OFF -Dimt=ON -Dccache=ON
How to customize builds

[phsft-bot]

Build failed on slc6/gcc62.
See console output.

Warnings:

• ninja: warning: multiple rules generate tutorials/hsimple.root. builds involving this target will not be correct; continuing anyway [-w dupbuild=warn]

Failing tests:

• projectroot.tree.treeplayer.test.gtest_tree_treeplayer_test_datasource_root

[pcanal]

Note that this does not solve the problem seen in roottest-root-io-json as the problem is actually that sometimes some elements of the vector are mis-detected as being allocated by TObject::operator new (because the memory allocated by the vector happens to be filled with the magic pattern TObject::operator new).

https://epsft-jenkins.cern.ch/job/root-nightly-master/BUILDTYPE=Release,COMPILER=native,LABEL=mac1012/1657/testReport/projectroot.roottest.root.io/json/roottest_root_io_json_make/

[pcanal]

However since json-io does not yet fully support TRef, I am merging this nonetheless.