The basic answer is that aphantasia is very real and is recognised and researched. It's just that not everything that is real can be tested directly.
There are some studies that have been done, but it is inherently difficult to study what happens inside a person's mind. This study reported that the participants occasionally had involuntary images in their mind and could dream in images, however they could not willingly create images. Another is here and discusses congenital aphantasia. This study is currently underway and is investigating both ends of the visualisation spectrum. Still, I have yet to find a way to measure it exactly in the way you are asking - there is no objective test because for those of us who experience it there are no images to test. It is not a matter of quality of visualisation, how much we can visualise, it's literally no visualisation.
There is one place trying to find a way to measure and manipulate visuals and their information is here. They have analysed brain activity when someone is using their mind's eye and are working on ways to manipulate the activity levels. They hope this can be particularly beneficial to those who are distracted by over active visualisations, but they are also looking into the applications for those with aphantasia. Again, though, this is only a measure of brain activity in certain areas and the size of certain regions of the brain. It's not a test as such.