Add double negative trait test case

Author: spastorino

src/test/ui/coherence/coherence-overlap-double-negative.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![feature(negative_impls)]
+#![feature(with_negative_coherence)]
+
+trait A {}
+trait B: A {}
+
+impl !A for u32 {}
+impl !B for u32 {}
+
+fn main() {}