Update pytest instructions in documentation (#130)

* Update pytest instructions in documentation

* Address PR comments

Author: mcbarton

CONTRIBUTING.md

@@ -54,6 +54,6 @@ cd ./test
 and the python tests by executing
 
 ```bash
-cd ./test
-pytest -sv .
+cd ../../test
+pytest -sv test_xcpp_kernel.py
 ```

README.md

@@ -52,7 +52,8 @@ cd test
 ```
 and
 ```bash
-pytest -sv .
+cd ../../test
+pytest -sv test_xcpp_kernel.py
 ```
 to perform the python tests.