Migrate #![no_std] tests to wasm

This required getting a little creative in a few places, but otherwise these
tests shouldn't need the test builder any more.

tests/no-std/test.rs

@@ -0,0 +1,28 @@
+//! This is a test that we compile `wasm-bindgen` itself in `no_std` mode and we
+//! can export/import various items.
+//!
+//! This doesn't actually run any tests, it's mostly a compile-time verification
+//! that things work.
+
+#![feature(use_extern_macros)]
+#![no_std]
+#![allow(dead_code)]
+
+extern crate wasm_bindgen;
+extern crate std as _some_other_name;
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+pub fn foo(_a: u32) {}
+
+#[wasm_bindgen]
+extern {
+    fn test(a: &str);
+
+    type Js;
+    #[wasm_bindgen(constructor)]
+    fn new() -> Js;
+    #[wasm_bindgen(method, structural)]
+    fn init(this: &Js);
+}