In standard English, many people might regard this as ill-formed, as the finite verb can requires a subject. However, things are not actually so simple. In fact, there are quite a few finite clauses even in standard English that do not have a subject.First, there are imperatives. For example, "Stand up" and "Come here" are both perfectly well formed, but neither has an overt subject.
Next, expressives often omit the subject. For instance, "Hope to see you soon" is a formal expressive that has no subject.
Then with ellipsis, it is possible for the subject to be omitted in conjoined clauses. In "John finished his meal and then went home", the second clause has no subject, because it has undergone ellipsis.
Finally, diary style often omits subjects: "Got up, ate breakfast."
With so many exceptions, it is not surprising if the possibility of a subjectless clause gets extended to "Can test here", especially in new varieties of English in places such as Brunei. In fact, I suspect this kind of sign might also sometimes occur in the UK and USA. So perhaps it is not so ungrammatical after all.
