Port testsuite to py.test

As per https://github.com/pocoo/metaflask/issues/8

  - [x] Make testsuite discoverable for py.test
    - [x] Move testsuite from ``/werkzeug/testsuite`` to ``/tests``
    - [x] Prefix ``.py``-files with ``test_``
  - [x] Remove all traces of custom testrunner code
  - [x] Remove usage of assertion methods
  - [x] Replace setUp and tearDown with fixtures
  - [x] Remove useless ``Test...``-classes in testsuite. Leaving them where subclassing is used for testing against multiple interfaces, and where they provide some form of separation, but no common superclass other than ``object`` anymore.