fixup: switch test from substring to regex

Author: JakobJingleheimer

test/es-module/test-esm-experimental-warnings.mjs

@@ -1,6 +1,6 @@
 import { mustCall } from '../common/index.mjs';
 import { fileURL } from '../common/fixtures.mjs';
-import { ok, strictEqual } from 'assert';
+import { match, strictEqual } from 'assert';
 import { spawn } from 'child_process';
 import { execPath } from 'process';
 
@@ -25,7 +25,7 @@ for (
   child.on('close', mustCall((code, signal) => {
     strictEqual(code, 0);
     strictEqual(signal, null);
-    ok(stderr.includes('ExperimentalWarning:'));
-    ok(stderr.includes(experiment), new Error(`Expected warning to mention ${experiment}`));
+    match(stderr, /ExperimentalWarning:/);
+    match(stderr, new RegExp(experiment));
   }));
 }