The params field is going to come back parsed into the correct ruby data structures (hash, array, string, etc). There's no need to manually split items such as +
, if there is a nested param it will return as part of the params object:
{tags: ["sea", "ship", "sun"]}
To access your data, or create an assumption about your param data existing in the test, you're going to want to create a stub. You're almost there, try something more along the lines of:
helper.stub!(:params).and_return({tags: ["sea", "ship", "sun"]})
Once you have the params stubbed correctly you can check the output of your helper method to ensure it's validity (this is called the expectation):
expect(helper.tags_list_with_destroy_links).to eq("some_url")