Add telemetry for setting python.testing.pytestPath and possibly remove it

[d3r3kk]

With the upcoming change to how we parse Pytest results #4035 we will also have to create a new way to deal with alternate configured Pytest paths.

We will need to update PYTHON_PATH to allow for the loading of Pytest from this custom location during the execution of our test discovery adapter.

Here's the original line we are talking about:
https://github.com/Microsoft/vscode-python/blob/0d565510ba16c133a61e59570904ceaeb42ce034/src/client/unittests/common/runner.ts#L93

There, we will have to implement something of this nature:

if there is a custom pytest path specified,
discover where pytest is being run from (pytest --version should tell us)
parse out the import path reuqired
put the import path at the beginning of PYTHONPATH
add the PYTHONPATH variable to the executed script spawn process

[DonJayamanne]

• If 'python.pytestPath' is set then:
  • pass it to the adapter script
  • In python code:
    • Run <pytestPath> --version
    • Get first path from the output
    • Set sys.path[0] to that (before importing pytest)
• Add telemetry to check how many people use python.pytestPath, python.nosePath

[karrtikr]

Set sys.path[0] to that (before importing pytest)

The problem with this approach is that adding that directory would import all the packages present in the directory other than pytest also, which conflicts with packages user has in their environment. Which basically means that with the current infrastructure, we cannot support python.testing.pytestPath reliably for discovery.

Leaving that setting there can be misleading, so my suggestion is to add telemetry to see if there are ppl using it - and then possibly remove it.

[luabud]

Dupe of #5820.