Run CI tests using cargo-nextest. (#400)

This brings the CI script in sync with the latest Linebender standard.

.github/workflows/ci.yml

@@ -34,6 +34,10 @@ env:
 # Using cargo-hack also allows us to more easily test the feature matrix of our packages.
 # We use --each-feature & --optional-deps which will run a separate check for every feature.
 #
+# We use cargo-nextest, which has a faster concurrency model for running tests.
+# However cargo-nextest does not support running doc tests, so we also have a cargo test --doc step.
+# For more information see https://github.com/nextest-rs/nextest/issues/16
+#
 # The MSRV jobs run only cargo check because different clippy versions can disagree on goals and
 # running tests introduces dev dependencies which may require a higher MSRV than the bare package.
 #
@@ -159,13 +163,21 @@ jobs:
         with:
           toolchain: ${{ env.RUST_STABLE_VER }}
 
+      - name: install cargo-nextest
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest
+
       - name: restore cache
         uses: Swatinem/rust-cache@v2
         with:
           save-if: ${{ github.event_name != 'merge_group' }}
 
-      - name: cargo test
-        run: cargo test --workspace --locked --all-features
+      - name: cargo nextest
+        run: cargo nextest run --workspace --locked --all-features --no-fail-fast
+
+      - name: cargo test --doc
+        run: cargo test --doc --workspace --locked --all-features --no-fail-fast
 
   test-stable-wasm:
     name: cargo test (wasm32)