Add tests to lower-constraints job

The current job simply installs requirements constrained by
lower-constraints.txt.  While it is nice to know that the
requirements files are compatible, it would be even better to
run some tests to actually check that the code runs using the
current lower-constraints.txt file.

Change-Id: I7a191ac44b004f840c986ded61341e43b35f1d49

tox.ini

@@ -199,3 +199,6 @@ deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
+commands =
+   {[testenv]commands}
+   stestr run --slowest {posargs}