Add unit tests for linter (#9)

.github/workflows/ci.yaml

@@ -73,6 +73,28 @@ jobs:
             ${{ runner.os }}-
       - run: cargo clippy -- -D warnings
 
+  cargo_test:
+    name: "cargo test"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+      - uses: actions/cache@v3
+        env:
+          cache-name: cache-cargo
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-build-${{ env.cache-name }}-
+            ${{ runner.os }}-build-
+            ${{ runner.os }}-
+      - run: cargo test
+
   maturin_build:
     name: "maturin build"
     runs-on: ubuntu-latest