Add missing test requirements, fixup pylint env

Add some missing test requirements.  I noticed these because pylint
was unhappy about the imports if you look closely.

Also, pylint shouldn't be in deps as it comes from the parent's
"test-requirements.txt" install.  We don't need the VIRTUAL_ENV
setting either.

Change-Id: Ie082a058a9d3d51164448410a00d0719b0b37c4a

test-requirements.txt

@@ -4,8 +4,11 @@
 hacking<0.11,>=0.10.0
 pylint==1.4.5 # GPLv2
 
+fixtures>=3.0.0 # Apache-2.0/BSD
+mock>=2.0 # BSD
 oslotest>=1.10.0 # Apache-2.0
 testrepository>=0.0.18 # Apache-2.0/BSD
+testtools>=1.4.0 # MIT
 
 # Doc requirements
 sphinx!=1.6.1,>=1.5.1 # BSD

tox.ini

@@ -17,8 +17,6 @@ commands =
   dib-lint
 
 [testenv:pylint]
-setenv = VIRTUAL_ENV={envdir}
-deps = pylint
 commands = pylint --rcfile pylint.cfg diskimage_builder
 
 [testenv:venv]