runtime_adapter.py: on timeout, use SIGTERM to stop the child

XXX what to do for non-unix platforms?

Author: yamt

test-runner/wasi_test_runner/runtime_adapter.py

@@ -42,15 +42,20 @@ def run_test(
             + [e for env in self._env_to_list(env_variables) for e in ("--env", env)]
         )
 
-        result = subprocess.run(
+        p = subprocess.Popen(
             args,
-            capture_output=True,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
             text=True,
-            check=False,
             cwd=Path(test_path).parent,
-            timeout=3.
         )
-        return Output(result.returncode, result.stdout, result.stderr)
+        try:
+            out, err = p.communicate(timeout=3)
+        except subprocess.TimeoutExpired:
+            p.terminate()
+            p.wait()
+            raise
+        return Output(p.returncode, out, err)
 
     @staticmethod
     def _abs(path: str) -> str: