test: add coverage for FSWatcher exception

Cover an previously uncovered exception possible in the internal start
function for FSWatcher.

Signed-off-by: Rich Trott <rtrott@gmail.com>

PR-URL: nodejs#32057
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>

Author: Trott

test/sequential/test-fs-watch.js

@@ -125,9 +125,31 @@ tmpdir.refresh();
       w.close();
     },
     {
+      name: 'Error',
+      code: 'ERR_INTERNAL_ASSERTION',
       message: /^handle must be a FSEvent/,
+    }
+  );
+  oldhandle.close(); // clean up
+}
+
+{
+  let oldhandle;
+  assert.throws(
+    () => {
+      const w = fs.watch(__filename, common.mustNotCall());
+      oldhandle = w._handle;
+      const protoSymbols =
+        Object.getOwnPropertySymbols(Object.getPrototypeOf(w));
+      const kFSWatchStart =
+        protoSymbols.find((val) => val.toString() === 'Symbol(kFSWatchStart)');
+      w._handle = {};
+      w[kFSWatchStart]();
+    },
+    {
       name: 'Error',
       code: 'ERR_INTERNAL_ASSERTION',
+      message: /^handle must be a FSEvent/,
     }
   );
   oldhandle.close(); // clean up