Add a tox target for unit tests with driver libraries

We mock out driver libraries in unit tests if they are not installed,
which leads to a periodic break in unit tests when they are actually
installed in the environment. This is sometimes a real break and
sometimes just a bad test, but at any rate it slows down packagers
and actual users.

Add a new tox target that runs unit tests with these installed,
so that we can put something in CI and stop breaking this.

Partial-Bug: #1626106
Change-Id: I81ce147c8958bb89b316c3c28fc9bce172f93873

tox.ini

@@ -16,6 +16,10 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
+[testenv:unit-with-driver-libs]
+deps = {[testenv]deps}
+    -r{toxinidir}/driver-requirements.txt
+
 [testenv:genstates]
 deps = {[testenv]deps}
     pydot2