CI: use current tests/ directory for upgrade testing

We should not rollback our test setup during upgrade test.
The only reason to do that would be for incompatible changes in the test
inventory, and we already checkout master for those (${CI_JOB_NAME}.yml)

Also do some cleanup by removing unnecessary intermediary variables

tests/scripts/testcases_run.sh

@@ -18,10 +18,9 @@ fi
 # Check out latest tag if testing upgrade
 if [ "${UPGRADE_TEST}" != "false" ]; then
   git fetch --all && git checkout "$KUBESPRAY_VERSION"
-  # Checkout the CI vars file so it is available
-  git checkout "${CI_COMMIT_SHA}" tests/files/${CI_JOB_NAME}.yml
-  git checkout "${CI_COMMIT_SHA}" ${CI_TEST_REGISTRY_MIRROR}
-  git checkout "${CI_COMMIT_SHA}" ${CI_TEST_SETTING}
+  # Checkout the current tests/ directory ; even when testing old version,
+  # we want the up-to-date test setup/provisionning
+  git checkout "${CI_COMMIT_SHA}" -- tests/
 fi
 
 # needed for ara not to complain
@@ -57,9 +56,8 @@ shift
 # We can set --limit here and still pass it as supplemental args because `--limit`  is a 'last one wins' option
 ansible-playbook \
      $ANSIBLE_LOG_LEVEL \
-    -e @${CI_TEST_SETTING} \
-    -e @${CI_TEST_REGISTRY_MIRROR} \
-    -e @${CI_TEST_VARS} \
+    -e @tests/common_vars.yml \
+    -e @tests/files/${CI_JOB_NAME}.yml \
     -e local_release_dir=${PWD}/downloads \
     "$@" \
     ${playbook}