Ubuntu 16.04 "xenial xerus" does not come with Python 2.x by default.
It is possible to install a python-2.7 package, but this has its own
problem: no `python` or `python2` symlink (see #7717).
This fixes the following scripts to work with python 3:
- `make check` (bctest,py, bitcoin-util-test.py)
- `make translate` (extract_strings_qt.py)
- `make symbols-check` (symbol-check.py)
- `make security-check` (security-check.py)
Explicitly call the python commands using $(PYTHON) instead
of relying on the interpreter line at the top of the scripts.
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
