tests: enable running wasm32-unknown-unknown tests (#4421)

* Several of tokio's features (e.g. the channel implementation) do not
  need a runtime to work, and can be compiled and used for
  wasm32-unknown-unknown targets
* This change enables running tests for the `sync` and `macros` features
  so that we can note any regressions there

.github/workflows/ci.yml

@@ -36,6 +36,7 @@ jobs:
       - loom-compile
       - check-readme
       - test-hyper
+      - wasm32-unknown-unknown
     steps:
       - run: exit 0
 
@@ -391,3 +392,17 @@ jobs:
           echo 'tokio-test = { path = "../tokio-test" }' >>Cargo.toml
           git diff
           cargo test --features full
+
+  wasm32-unknown-unknown:
+    name: test tokio for wasm32-unknown-unknown
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install Rust
+        run: rustup update stable
+      - uses: Swatinem/rust-cache@v1
+      - name: Install wasm-pack
+        run: curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh
+      - name: test tokio
+        run: wasm-pack test --node -- --features "macros sync"
+        working-directory: tokio