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

.github/workflows/CI.yml

@@ -64,12 +64,11 @@ jobs:
       run: cargo hack check --each-feature --no-dev-deps --all
 
   test-versions:
-    # Test against the stable, beta, and nightly Rust toolchains on ubuntu-latest.
     needs: check
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [stable, beta, nightly, 1.54]
+        rust: [stable, beta, nightly]
     steps:
     - uses: actions/checkout@master
     - uses: actions-rs/toolchain@v1
@@ -84,6 +83,28 @@ jobs:
         command: test
         args: --all --all-features --all-targets
 
+  # some examples doesn't support 1.54 (such as async-graphql)
+  # so we only test axum itself on 1.54
+  test-msrv:
+    needs: check
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust: [1.54]
+    steps:
+    - uses: actions/checkout@master
+    - uses: actions-rs/toolchain@v1
+      with:
+        toolchain: ${{ matrix.rust }}
+        override: true
+        profile: minimal
+    - uses: Swatinem/rust-cache@v1
+    - name: Run tests
+      uses: actions-rs/cargo@v1
+      with:
+        command: test
+        args: -p axum --all-features --all-targets
+
   test-docs:
     needs: check
     runs-on: ubuntu-latest