Reorganize tests (#456)

* Reorganize tests

This breaks up the large `crate::tests` module by moving some of the
tests into a place that makes more sense. For example tests of JSON
serialization are moved to the `crate::json` module. The remaining routing
tests have been moved to `crate::routing::tests`.

I generally prefer having tests close to the code they're testing. Makes
it easier to see how/if something is tested.

* Try pinning to older version of async-graphql

* Revert "Try pinning to older version of async-graphql"

This reverts commit 2e2cae7d12f5e433a16d6607497d587863f04384.

* don't test examples on 1.54 on CI

* move ci steps around a bit

src/error_handling/mod.rs

@@ -203,7 +203,7 @@ pub mod future {
 
 #[test]
 fn traits() {
-    use crate::tests::*;
+    use crate::test_helpers::*;
 
     assert_send::<HandleError<(), (), NotSendSync>>();
     assert_sync::<HandleError<(), (), NotSendSync>>();