Fix bug 1680 to run stage-test only if changes in installer charms

Change-Id: Ibf372557be7de1ce40461315b0fdb40f8f0ad1ca
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>

devops-stages/stage-test.sh

 #!/bin/sh
 #   Copyright 2021 Canonical Ltd.
+#   Copyright ETSI
 #
 #   Licensed under the Apache License, Version 2.0 (the "License");
 #   you may not use this file except in compliance with the License.
@@ -14,6 +15,11 @@
 #   limitations under the License.
 
 set -eu
+
+if [ $(git diff --name-only origin/$GERRIT_BRANCH -- installers/charm/ |wc -l) -eq 0 ]; then
+    exit 0
+fi
+
 CURRENT_DIR=`pwd`
 
 # Execute tests for charms
@@ -23,4 +29,4 @@ for charm in $CHARM_NAMES; do
     cd $CHARM_PATH/$charm
     TOX_PARALLEL_NO_SPINNER=1 tox --parallel=auto
     cd $CURRENT_DIR
-done
\ No newline at end of file
+done