srml-contract: test calls not dispatched if tx fails (#2917)

* Test for not dispatching calls if top level execution fails

* Add comment to test

* Only dispatch calls if contract execution succeeded

Note that `calls` should be empty in this case, but this makes things
clearer

* Add comment to test

Co-Authored-By: Sergei Pepyakin <[email protected]>

* Revert: Only dispatch calls if contract execution succeeded