Test suite shouldn't depend on python-fixtures

[Martin Packman]

As of r124 the test suite fails to import if fixtures is not found:

Traceback (most recent call last):
  File "C:\bzr\testtools\testtools\run.py", line 280, in ?
    program = TestProgram(argv=sys.argv, testRunner=runner)
  File "C:\bzr\testtools\testtools\run.py", line 133, in __init__
    self.parseArgs(argv)
  File "C:\bzr\testtools\testtools\run.py", line 188, in parseArgs
    self.createTests()
  File "C:\bzr\testtools\testtools\run.py", line 197, in createTests
    self.module)
  File "C:\Python24\lib\unittest.py", line 556, in loadTestsFromNames
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "C:\Python24\lib\unittest.py", line 544, in loadTestsFromName
    test = obj()
  File "C:\bzr\testtools\testtools\tests\__init__.py", line 9, in test_suite
    from testtools.tests import (
  File "C:\bzr\testtools\testtools\tests\test_testtools.py", line 9, in ?
    import fixtures
ImportError: No module named fixtures