Migrate to trybuild from compiletest

This migrates our UI tests to the recently published `trybuild` crate
which should hopefully be a bit more robust than `compiletest` over
time!

crates/macro/tests/ui.rs

@@ -0,0 +1,5 @@
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("ui-tests/*.rs");
+}