The conjunction in the sentence is "unless" and it is a subordinating conjunction.
Firstly, a conjunction is basically a word to connect two clauses in a sentence. A subordinating conjunction is specifically a conjunction ("unless" in this example) that connects and shows the relationship between an independent clause ("we will test our invention outdoors,") and dependent clause ("it rains"). This indicates that rain is a factor that will determine whether or not the speaker will test an invention outdoors.