better method for detecting lab for tests

[scottdraves]

https://github.com/twosigma/beakerx/blob/master/test/run_tests.py#L35

instead of looking for the name of the conda env, just use a cmd line parameter to the script.

run_tests.py lab => run lab tests.
run_tests.py nb => run notebook tests.

if neither argument is given, then run "conda list" and look for "jupyterlab" in the results.

the notebook tests should also pass if explicitly run even if lab is installed.