Clean up tests for Z2 ZPT expresssion machinery:

o Move unneeded global imports to point of use.

o Wrap long lines.

o Make setUp / tearDown explicit.

o Avoid use of testcase instance variables.

o Split apart edge cases into discrete testcase methods.

o Use stock 'assertEqual', 'failUnless' methods of unittest.

Test both the 'trusted' and the 'untrusted' versions of the engine,
using a common base class.

  XXX:  This split will allow new tests which verify the security
        semantics of each version of the engine.