ci: use feature matrix for cargo test

Adopts a similar feature matrix approach as the asn1-rs CI. `cargo test`
will be run for no default features, default features, and all features.

The asn1-rs matrix differs only by having a separate "no std" job that
uses `--no-default-features`. Since this crate doesn't offer a std
feature we roll this step into the same matrix as the other feature
configs.

.github/workflows/rust.yml

@@ -29,12 +29,17 @@ jobs:
   test:
     name: Test Suite
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        features:
+          - --no-default-features
+          - --features=default
+          - --all-features
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
-      - run: cargo test
-      - run: cargo test --all-features
-      - run: cargo test --no-default-features
+      - name: Install stable toolchain
+        uses: dtolnay/rust-toolchain@stable
+      - run: cargo test ${{ matrix.features }}
 
   fmt:
     name: Rustfmt