Avoid using the py.test wrapper and directly call python2 -m pytest

As it might not be installed in some systems.

Changelog: None

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

scripts/run-tests.sh

@@ -77,7 +77,7 @@ convert_and_test() {
 
     cd ${WORKSPACE}/mender-image-tests
 
-    py.test --verbose \
+    python2 -m pytest --verbose \
             --junit-xml="${WORKSPACE}/results.xml" \
             --test-conversion \
             --test-variables="${MENDER_CONVERT_DIR}/deploy/${device_type}-${artifact_name}.cfg" \