bk test: support `bk test --assert` which will turn on Python assertions

This is off by default, as it would cause html parsing to fail.

(integrated from komodo (trunk) change 84094 by marky)

git-svn-id: http://svn.openkomodo.com/repos/openkomodo/trunk@14260 b87d16e7-d29f-4ee1-b5ec-817d645d335f