test: simplify test-worker-syntax-error

Remove extraneous code from test-worker-syntax-error. Because the worker
is called with `eval: true`, there is no need to set an environment
variable indicating whether the worker has started and so on. The test
file is only ever executed by the main thread.

PR-URL: #26144
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-worker-syntax-error.js

@@ -3,15 +3,9 @@ const common = require('../common');
 const assert = require('assert');
 const { Worker } = require('worker_threads');
 
-// Do not use isMainThread so that this test itself can be run inside a Worker.
-if (!process.env.HAS_STARTED_WORKER) {
-  process.env.HAS_STARTED_WORKER = 1;
-  const w = new Worker('abc)', { eval: true });
-  w.on('message', common.mustNotCall());
-  w.on('error', common.mustCall((err) => {
-    assert.strictEqual(err.constructor, SyntaxError);
-    assert(/SyntaxError/.test(err));
-  }));
-} else {
-  throw new Error('foo');
-}
+const w = new Worker('abc)', { eval: true });
+w.on('message', common.mustNotCall());
+w.on('error', common.mustCall((err) => {
+  assert.strictEqual(err.constructor, SyntaxError);
+  assert(/SyntaxError/.test(err));
+}));