Add trybuild tests for axum-debug (#501)

The things are also covered by the doc tests but this also gives us
assertions on the errors produced, which is the whole point of this
crate :)

axum-debug/tests/fail/not_a_function.rs

@@ -0,0 +1,6 @@
+use axum_debug::debug_handler;
+
+#[debug_handler]
+struct A;
+
+fn main() {}