qa: Prepare functional tests for Windows
Summary:
* Pass `sys.executable` when calling a python script via the subprocess module * Don't remove the log file while it is still open and written to * Properly use os.pathsep and os.path.sep when modifying the PATH environment variable * util-tests: Use os.path.join for Windows compatibility
Backport of core PR11858
https://github.com/bitcoin/bitcoin/pull/11858/files
Include a fix from PR12438:
https://github.com/bitcoin/bitcoin/pull/12438/files
Test Plan:
make check ./test/functional/test_runner.py
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3499