Add a proc-macro UI test example to the test project

Taken from work on rust-lang/rust-clippy#3741.

I think this is a useful test to have because it tests

* the `run-pass` header
* the `aux-build` header, building auxiliary files
* the `no-prefer-dynamic` header
* proc-macro crate interactions with all of the above
* running of UI tests that include auxiliary files

There's no tests for any of the above right now.

test-project/tests/ui/auxiliary/simple_proc_macro.rs

@@ -0,0 +1,12 @@
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::{TokenStream};
+
+#[proc_macro]
+pub fn macro_test(input_stream: TokenStream) -> TokenStream {
+    TokenStream::new()
+}

test-project/tests/ui/some-proc-macro.rs

@@ -0,0 +1,12 @@
+// run-pass
+// aux-build:simple_proc_macro.rs
+
+#![feature(proc_macro_hygiene)]
+
+extern crate simple_proc_macro;
+use simple_proc_macro::macro_test;
+
+fn main() {
+    println!("hi");
+    macro_test!(2);
+}