Added running pytest in tox

Signed-off-by: Grzegorz Latosinski <glatosinski@antmicro.com>

tox.ini

diff --git a/tox.ini b/tox.ini
index 0979f90..7fcb0fc 100644
--- a/tox.ini
+++ b/tox.ini
@@ -22,6 +22,7 @@
     # use `python setup.py check -m -r -s` instead.
     python setup.py check -m -s
     flake8 .
+    pytest
 [flake8]
 exclude = .tox,*.egg,build,data
 select = E,W,F