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_async.stderr

@@ -0,0 +1,5 @@
+error: handlers must be async functions
+ --> tests/fail/not_async.rs:4:1
+  |
+4 | fn handler() {}
+  | ^^