lib: make internal API warning more direct

Before:
These APIs are exposed only for testing and are not tracked by any
versioning system or deprecation process.

After:
These APIs are for internal testing only. Do not use them.

PR-URL: #25125
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/internal/test/binding.js

@@ -1,8 +1,7 @@
 'use strict';
 
 process.emitWarning(
-  'These APIs are exposed only for testing and are not ' +
-  'tracked by any versioning system or deprecation process.',
+  'These APIs are for internal testing only. Do not use them.',
   'internal/test/binding');
 
 module.exports = { internalBinding };

lib/internal/test/heap.js

@@ -1,8 +1,7 @@
 'use strict';
 
 process.emitWarning(
-  'These APIs are exposed only for testing and are not ' +
-  'tracked by any versioning system or deprecation process.',
+  'These APIs are for internal testing only. Do not use them.',
   'internal/test/heap');
 
 const { createHeapDump, buildEmbedderGraph } = internalBinding('heap_utils');

test/parallel/test-dns-lookup.js

@@ -24,8 +24,7 @@ cares.getaddrinfo = () => internalBinding('uv').UV_ENOENT;
 common.expectWarning({
   // For 'internal/test/binding' module.
   'internal/test/binding': [
-    'These APIs are exposed only for testing and are not ' +
-    'tracked by any versioning system or deprecation process.'
+    'These APIs are for internal testing only. Do not use them.'
   ],
   // For dns.promises.
   'ExperimentalWarning': [