Disable ui tests on emscripten

These hit a strange error in our Emscripten builder in Travis as well as
locally through cargo web:

    ERROR: failed to execute cargo: Resource temporarily unavailable (os error 11)

test_suite/tests/compiletest.rs

@@ -1,3 +1,4 @@
+#[cfg(not(target_os = "emscripten"))]
 #[rustc::attr(not(nightly), ignore)]
 #[test]
 fn ui() {