AutoTest: subtyping qml TestCase should preserve TestCase nodes in Quick Tests tree

Steps to reproduce:
create a new qml type on top of TestCase
create two tst_*.qml documents with that type as root and add at least a test_ function for each document

Current result:
Tree in test pane does not show the names of the two test cases, all test functions are under the same root node

Note: not sure the limitation is only of the autotest plugin, even running the qmake .pro on command line seemed to give me a single context for both tst_ files, but that is probably a separate issue