Fix python cross tests

This fixes #670 (closed).

To test, make sure python is Python 2 and you have a python3. Then run:

# It is probably a good idea to start from a clean build tree:
# rm -rf build

./waf configure --python=python3
./waf build

On git master, you will get something like this:

  File "/home/rlaager/src/ntpsec/ntpsec/wscript", line 902, in bin_test
    from wafhelpers.bin_test import cmd_bin_test
  File "/home/rlaager/src/ntpsec/ntpsec/wafhelpers/bin_test.py", line 10, in <module>
    import ntp.util
  File "/home/rlaager/src/ntpsec/ntpsec/build/main/tests/pylib/ntp/util.py", line 16, in <module>
    import ntp.ntpc
ImportError: No module named ntpc

With this MR, the tests pass.

The first commit can be squashed into the second, if desired.

The third commit can be squashed into the fourth, too. I kept them separate to ease review.