move test that shouldn't be in test/run-pass/

Author: Centril

src/test/run-pass/generator/niche-in-generator.rs renamed to src/test/ui/generator/niche-in-generator.rs

@@ -1,5 +1,7 @@
 // Test that niche finding works with captured generator upvars.
 
+// run-pass
+
 #![feature(generators)]
 
 use std::mem::size_of_val;