tests: Make it possible to run functional tests on Windows

Summary:
Backport of core PR14007.

Depends on D5958.

Test Plan:

ninja check-functional

The output should be all green. Not blue.

Build for windows, then:

• Install python if needed and add it to your path
• If you copy the project to some place, you need at least the src,

test, share and your build directories.

• Copy the test_runner.py to the build directory to replace the symlink
• Update the config.ini file to match you paths

Then in a powershell:

$env:PYTHONIOENCODING="utf-8"
python .\test\functional\test_runner.py --force

Most of the tests should pass, but still far from all. More backports
coming.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5964