fix(c/driver_manager): test and fix bugs in profiles

[lidavidm]

• Fix substitution of undefined env var erasing the entire value instead of substituting in a blank string.
• Improve error messages.
• Add end-to-end tests with Python.
• Add end-to-end tests in Conda/virtualenv.
• Add end-to-end tests with the user path (~/.config, etc.)

This is not complete; see #4082, #4085, #4086, #4087 for things that also need to be fixed, but this establishes a baseline of tests.

Closes #4024.

[lidavidm]

@zeroshade @amoeba venvs don't work because we add the additional search path <venv root>/etc/adbc/drivers, and have no way to configure profiles.

Options are:

• Add the other search path; but then we search both paths for both manifests and profiles, and we risk confusing one for the other.
• We could additionally require profiles to have a different extension.
• Instead, we could add a second API function to set another search path. That's starting to get unwieldy.
• We could ditch the API function, and instead use normal driver options to set additional search paths for drivers/manifests.

[lidavidm]

I need to emphasize that this does not fix everything; see bugs linked above for other things we need to fix still.

[lidavidm]

next PR: #4088