pytest test discovery fails when pytest writes directly to process' stderr

[gqmelo]

Issue Type: Bug

Since the python discovery was revamped (I don't recall when), my test discovery stopped working with pytest, giving the following message:

python <HOME_PATH>/.vscode/extensions/ms-python.python-2019.10.41019/pythonFiles/testing_tools/run_adapter.py discover pytest -- -s --cache-clear -vvv -ra --color=no --no-cov test
Test Discovery failed: 
Error: 2019-10-23 11:24:14 <SOME MESSAGE FROM IN-HOUSE CODE>

The problem is caused by an internal pytest plugin that runs subprocess.check_output like this:

    ...
    stdout = subprocess.check_output(arguments)
    # Parse the output to discover some paths
    ...

This subprocess write some log messages to stderr and those messages go directly to the current process' stderr, not sys.stderr.

So when vscode runs the run_adapter.py script, the following code can't capture those log messages:

    with (util.hide_stdio() if hidestdio else util.noop_cm()) as stdio:
        ec = _pytest_main(pytestargs, [_plugin])

When running the run_adapter.py script manually I get:

2019-10-23 11:06:33 <SOME MESSAGE FROM IN-HOUSE CODE>
[<JSON_CONTENT>]

But if I run the script re-directing stderr (i.e. 2> /dev/null), I get only the json content.

So while we can fix this on our side by doing subprocess.check_output(arguments, stderr=sys.stderr), I believe it would be better if vscode tries to parse only run_adapter.py's stdout. Otherwise anyone using subprocess or doing something else that writes to the process' stderr can break vscode test discovery.

Extension version: 2019.10.41019
VS Code version: Code 1.39.2 (6ab598523be7a800d7f3eb4d92d7ab9a66069390, 2019-10-15T15:33:40.634Z)
OS version: Linux x64 4.15.0-65-generic

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-7600U CPU @ 2.80GHz (4 x 3441)
GPU Status	2d_canvas: enabled flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: disabled_software oop_rasterization: disabled_off protected_video_decode: unavailable_off rasterization: disabled_software skia_deferred_display_list: disabled_off skia_renderer: disabled_off surface_synchronization: enabled_on video_decode: unavailable_off viz_display_compositor: disabled_off webgl: enabled webgl2: enabled
Load (avg)	3, 3, 2
Memory (System)	15.55GB (0.15GB free)
Process Argv	--unity-launch
Screen Reader	no
VM	0%

[brettcannon]

Duplicate of #6594