test_nodebug broken if not running under tox

[int19h]

This is specifically for running those tests via VSCode after opening the debugpy workspace.

The problem is that it uses debuggee.backchannel, which tries to import a bunch of stuff from debugpy.common. And this fails, because debugpy itself cannot be imported, because it's not on PYTHONPATH.

We try to make sure that it's there by setting PYTHONPATH=./src in .env - but that only works for test discovery, not for debuggee processes spawned by the tests, because the latter run in a different directory, and thus the meaning of . changes.

The fundamental problem here is that PYTHONPATH needs to be set to an absolute path for it to work consistently, and yet there's no way to use ${workspaceFolder} etc inside .env, which is necessary for that to be set in a workspace-independent way; nor is there any other way to define environment variables with expansions like that for Run Test (Debug Test can use launch.json, at least). That's microsoft/vscode-python#11932 - but in the meantime, we can hack around this in tests by checking whether debugpy comes from site-packages or not; and if not, manually add it to PYTHONPATH for debuggee.