Issue 5354: Provide a standardised testing mechanism for doing fresh imports of modules, including the ability to block extension modules in order to test the pure Python fallbacks

its clarity and its speed.

- Add tests for the module-level load() and dump() functions.
- Add tests for cPickle's internal data structures, stressing workloads
with many gets/puts.
- Add tests for the Pickler and Unpickler classes, in particular the
memo attribute.
- test_xpickle is extended to test backwards compatibility with Python
2.4, 2.5 and 2.6 by round-tripping pickled objects through a worker
process. This is guarded with a regrtest -u xpickle resource.

- Break the gc cycle in negotiation tests
- test the different guarantees of read_lazy and read_very_lazy

Issue #5695: Minor tweak to improve the code as suggested by Brett Cannon and as implemented in the Py3K branch.

  now collapsed within the url properly before looking in cgi_directories.