Programmatically declare test dependencies.

This can be used e.g. 'pip install -e .[test]'

Change-Id: I51c0411af37ab2c43d068d22f0e3473dd958c7ba

doc/hacking.rst

@@ -55,6 +55,9 @@ methods are implemented. However, when writing tests for testtools itself, it
 is often useful to see all levels of the stack. To do this, add
 ``run_tests_with = FullStackRunTest`` to the top of a test's class definition.
 
+To install all the test dependencies, install the ``test`` extra::
+
+  pip install e .[test]
 
 Discussion
 ----------

setup.cfg

@@ -9,6 +9,11 @@ classifier =
     License :: OSI Approved :: MIT License
     Programming Language :: Python :: 3
 
+[extras]
+test =
+  fixtures
+  unittest2>=1.1.0
+
 [files]
 packages = testtools