worker: improve coverage

This improves the worker coverage by using `internal/assert` instead
of relying on `assert` in case a faulty worker message type is
received.

PR-URL: #27230
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

lib/internal/main/worker_thread.js

@@ -45,6 +45,8 @@ const {
 
 const publicWorker = require('worker_threads');
 
+const assert = require('internal/assert');
+
 patchProcessObject();
 setupInspectorHooks();
 setupDebugEnv();
@@ -122,18 +124,17 @@ port.on('message', (message) => {
       process.argv[1] = filename; // script filename
       require('module').runMain();
     }
-    return;
   } else if (message.type === STDIO_PAYLOAD) {
     const { stream, chunk, encoding } = message;
     process[stream].push(chunk, encoding);
-    return;
-  } else if (message.type === STDIO_WANTS_MORE_DATA) {
+  } else {
+    assert(
+      message.type === STDIO_WANTS_MORE_DATA,
+      `Unknown worker message type ${message.type}`
+    );
     const { stream } = message;
     process[stream][kStdioWantsMoreDataCallback]();
-    return;
   }
-
-  require('assert').fail(`Unknown worker message type ${message.type}`);
 });
 
 // Overwrite fatalException