test runner: stop if any step failed

There's no point in continuing if, say, spawning a process fails.  Also,
the validation that we perform for a given step depends on the previous
steps having succeeded.

Author: wingo

test-runner/wasi_test_runner/runtime_adapter.py

@@ -117,6 +117,9 @@ def run_test(self, config: Config, argv: List[str]) -> Result:
         cleanup_dirs = None
         try:
             for op in config.operations:
+                if result.failures:
+                    break
+
                 match op:
                     case Run(_, _, dirs):
                         _cleanup_test_output(dirs)