tests: fix run-spread test to allow running multiple tests on custom …

…variant (#13751)

* fix run-tests test to allow to run multiple tests on custom variant

Ths is needed to re-exec failed tests on beta validation

* add acho to show tests to be executed

* fix shellcheck disable=SC2086

* add stderr to spread.log

tests/nested/manual/run-spread/task.yaml

@@ -42,4 +42,6 @@ execute: |
         remote.refresh full
     fi
 
-    "$SPREAD" -v "$RUN_TESTS" | tee spread.log
+    echo "Running $SPREAD -v $RUN_TESTS"
+    # shellcheck disable=SC2086
+    "$SPREAD" -v $RUN_TESTS |& tee spread.log