Run tests sequentially: cargo test -j 1

I think it is possible that our signal handler set in the test can be
overwritten by integration tests (under dir 'test'), which means the
`SIGNALED` variable would never be set.

We have a lock to prevent this issue in integration tests, but since this
is a doc test, we cannot employ it here.

```rust
/// Any test that alters signal handling must grab this mutex.
pub static SIGNAL_MTX: Mutex<()> = Mutex::new(());
```

So let's run tests sequentially and see if it will work.

Author: SteveLauC

.github/actions/test/action.yml

@@ -34,4 +34,4 @@ runs:
 
     - name: test
       shell: bash
-      run: ${{ inputs.SUDO }} $(which ${{ inputs.TOOL }}) test --target ${{ inputs.TARGET }}
+      run: ${{ inputs.SUDO }} $(which ${{ inputs.TOOL }}) test --target ${{ inputs.TARGET }} -j 1