Test discovery should expand system variables in .env

[karthiknadig]

Environment data

• VS Code version: 1.38.1
• Extension version (available under the Extensions sidebar): 2019.9.34911
• OS and version: Windows
• Python version (& distribution if applicable, e.g. Anaconda): 3.7
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): N/A
• Relevant/affected Python packages and their versions: PyTest
• Jedi or Language Server? (i.e. what is "python.jediEnabled" set to; more info How to update the language server to the latest stable version #3977): LS

Expected behaviour

Should expand ${workspaceFolder}/src in .env while doing test discovery or test runs.
.env:

PYTHONPATH=${workspaceFolder}/src

Actual behaviour

Does not expand ${workspaceFolder} for test discovery.

Steps to reproduce:

1. Clone ptvsd, open it and wait for extension to load
2. Once you see tests are discovered.
3. Open .env file and change from PYTHONPATH=./src to PYTHONPATH=${workspaceFolder}/src
4. Reload.
5. Test discovery fails.

Logs

Python Test Logs.

python c:\Users\kanadig\.vscode\extensions\ms-python.python-2019.9.34911\pythonFiles\testing_tools\run_adapter.py discover pytest -- -s --cache-clear
Test Discovery failed: 
Error: ImportError while loading conftest 'c:\GIT\ptvsd\tests\conftest.py'.
tests\__init__.py:52: in <module>
    from ptvsd.common import json, log
E   ModuleNotFoundError: No module named 'ptvsd'

[DonJayamanne]

Duplicate of #7581

[DonJayamanne]

Sorry, this isn't a duplicate of #7581.
Closing this as ${workspaceFolder} and such variables are not supported in .env files.

.env files is not a concept that was coined by the VS Code extension, this is in use by other tools.
We won't be adding support for such variables that are specific to the context of the vs code execution environment as these will not work outside VS Code (making the .env file less portable and more VS Code specific (which they aren't)

@karthiknadig