tests: Move definition of topdir into new tests/env.py
tests: NFC line length tidy up
tests: Ensure templates can be expanded regardless of CWD
tests: Add crude command line for test_helper

list and build commands added

test_helper.py list:
  List all available images

test_helper.py build all:
  Build all images

test_helper.py build <img>
  Build a single image

Expansion parameters can be appended to build commands
tests: Add RHEL 4.6 i386 image