tests: fix python test-runner for windows

Windows needed a few fixups to get the tests running:
1. bitcoin-tx needs a file extension in Windows. Take this opportunity to
   add an env file, which pulls variables out of our build config. This can
   be extended as needed, for now it's very simple.
2. After #1, split the args out of the exec key in the test data.
3. Correct the line-endings from windows stdout

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