Remove testutils feature so doctests worka again

I wanted to only conditionally compile testutils but it's needed in
doctests which we can't conditionally compile for:

https://github.com/rust-lang/rust/issues/67295

src/lib.rs

@@ -262,5 +262,9 @@ pub fn version() -> &'static str {
     env!("CARGO_PKG_VERSION", "unknown")
 }
 
-#[cfg(any(feature = "testutils", test))]
+// We should consider putting this under a feature flag but we need the macro in doctets so we need
+// to wait until https://github.com/rust-lang/rust/issues/67295 is fixed.
+//
+// Stuff in here is too rough to document atm
+#[doc(hidden)]
 pub mod testutils;