Run ui tests on nightly only

Where the nightly and other compiler stderr messages differ, nightly is
the one that makes sense to test.

Cargo.toml

@@ -23,6 +23,7 @@ ryu = "0.2"
 
 [dev-dependencies]
 automod = "0.1"
+select-rustc = "0.1"
 serde_bytes = "0.11"
 serde_derive = "1.0"
 serde_stacker = "0.1"

tests/compiletest.rs

@@ -1,3 +1,4 @@
+#[rustc::attr(not(nightly), ignore)]
 #[test]
 fn ui() {
     let t = trybuild::TestCases::new();