fixup: switch test from substring to regex

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

@@ -1,15 +1,15 @@
 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';
 
 // Verify experimental warnings are printed
 for (
   const [experiment, arg] of [
-    ['Custom ESM Loaders', `--experimental-loader=${fileURL('es-module-loaders', 'hooks-custom.mjs')}`],
-    ['Network Imports', '--experimental-network-imports'],
-    ['Specifier Resolution', '--experimental-specifier-resolution=node'],
+    [/Custom ESM Loaders/, `--experimental-loader=${fileURL('es-module-loaders', 'hooks-custom.mjs')}`],
+    [/Network Imports/, '--experimental-network-imports'],
+    [/Specifier Resolution/, '--experimental-specifier-resolution=node'],
   ]
 ) {
   const child = spawn(execPath, [
@@ -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, experiment);
   }));
 }