She is trying to let you know that she is not that interested in the convo, or you for that matter. You may be boring her. It is a hint that a message may be more important than what you have to say...
Bullshlt!!!
...wait.. no I actually have to agree with you on this one.
I don't think this was a test. Think about it. Girls and Guys are not so unalike that they have 180 degree different behaviours to _everything_. In fact we act quite similar when we are expressing that we are bored. We act quite similar in more ways than people care to realize! I find people hiding behind the exucse of "women are crazy" instead of seeing the forest through the tress...usually due to attachment to expectations. But anyway I digress....
If a girl is going to test your patience or how easily you're put off balance by something annoying...she's going to do something a LOT bigger than look for messages on her cell phone during a conversation.
She's going to hide your wallet on you before you go to work to see how easily you get frustrated or upset with her in a tense situation...or something to that degree.
You'd be surprised how infrequest women REALLY conciously test you and how the pop-culture idea of "testing" actually gets in the way of the reality that's staring you right in the face. People like to call most of womans "**** behaviour" testing because it gives you an easy finger to point and an immidiate way to react to a situation, but not always the best way.
Real tests usually only come after a girl decides she likes you and is thinking about being with you to some degree, and are they REALLY OBVIOUS, because they will be things that she does that are far out of context from her usual behaviour.
If a girl is checking her cell while you're having a conversation it's probably because you're not being interesting.
DJHoolahoops suggestion may have worked since it would have added some emotion to the situation = more interesting. It would have thrown her out of her bored state fairly qucikly and given you the opportunity to build from base level again.