Ok, so let me describe what a test is..
In rough english it would be:
If condition
.... do something
else
... do something else
endif
If your case you said you only wanted to add the rel="noopener" if the target was a new window (i.e. "_blank")
So in TWIG language you would write that as
{% if item.target|default('_blank') == "_blank" %}
rel="noopener"
{% endif %}
So that means that the code that you provided should be:
<a target="{{ item.target|default('_blank') }}" href="{{ item.link|e }}" {% if item.target|default('_blank') == "_blank" %}rel="noopener"{% endif %} title="{{ item.title }}">{{ item.title|raw }}</a>
Also, the comparison to an editor is not really applicable here since there is no ONE editor for particles - every particle had it's own code - so every particle would need to be changed to add this functionality for links. Also, the assertion that everyone will have the same requirement as you to add rel="noopener" to every link that opens in a new window may be incorrect. So the suggestion that I have actually made for future particles is that we just add a "rel" field into which you can key what you like as a user and it will be added to the link. In fact, we did already do something very similar for the Gantry 5 menu editor where you can add attributes to the link.
I hope that now helps.
Regards, Mark.