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/argument_not_extractor.stderr

@@ -0,0 +1,11 @@
+error[E0277]: the trait bound `bool: FromRequest` is not satisfied
+ --> tests/fail/argument_not_extractor.rs:4:23
+  |
+4 | async fn handler(foo: bool) {}
+  |                       ^^^^ the trait `FromRequest` is not implemented for `bool`
+  |
+note: required by a bound in `handler::{closure#0}::debug_handler`
+ --> tests/fail/argument_not_extractor.rs:4:23
+  |
+4 | async fn handler(foo: bool) {}
+  |                       ^^^^ required by this bound in `handler::{closure#0}::debug_handler`