test: test_python: set PYTHONPATH to the python execution environment

Place PYTHONPATH to the environment when python is executed in a way
that current shell environment is not affected. This also allows adding
the old value of PYTHONPATH to the end of the new value (otherwise it
would have been appended again and again when test_python is called).

At the same time, use -B option to avoid writing .pyc files to
bindings/python/* (which are not cleared out by distclean).

Drop the (unused) prefix code which preserved the original stdout of the
python program and opened sys.stdout to OUTPUT. In place of that there
is now note how (debug) information can be printed to original stdout.