Extract graph schema validation into properly named test

[T4rk1n]

In test: test_tagr002_tabs_render_without_selected we do

    # do some extra tests while we're here
    # and have access to Graph and plotly.js
    check_graph_config_shape(dash_dcc)

This check that we have all the props defined, but the test is random and parametrized so the same check is run twice. It should be in it's own test.