Add tests for stackTrace, scopes, variables

[jtpio]

Add several tests for stackTrace, scopes and variablesReference.

They can also serve as a reference for the protocol messages.

[jtpio]

This change also adds an execute method to the DebugSession, used to send a request_execute message to the kernel and start debugging the code.

[jtpio]

This could be also rebased on #15 if we get it in first.

[afshin]

Hm, the tests hang for me:

[jtpio]

Looks like it cannot find xeus-python and start it.

[jtpio]

Maybe we should make the tests install the spec for xpython to the temp data_dir instead: https://github.com/jupyterlab/jupyterlab/blob/f1bc02ced61881df94c49929837c49c022f5b115/jupyterlab/tests/test_jupyterlab.py#L126

[afshin]

That sounds like a good idea. Hm, I have the kernel when I run lab ...

[jtpio]

The last commit installs the kernel spec for xeus-python automatically when running the tests.

Opened jupyterlab/jupyterlab#7089 to make it easier to do the same in other third-party extensions.

[jtpio]

@afshin @KsavinN what do you think about getting this one in?

Then we can take care of #32 and start integrating the debug messages with the UI.

[afshin]

👍