Update test harness for browser testing

This commit updates the test harness for in-browser testing. It now no longer
unconditionally uses `fs.writeSync`, for example. Instead a `Formatter` trait is
introduced for both Node/browser environments and at runtime we detect which is
the appropriate one to use.

crates/test/src/lib.rs

@@ -20,6 +20,6 @@ macro_rules! console_log {
     )
 }
 
-#[path = "rt.rs"]
+#[path = "rt/mod.rs"]
 #[doc(hidden)]
 pub mod __rt;