test: fix flaky test-heapdump-http2

Replace stream.respondWithFile(__filename) with
stream.respondWithFile(process.execPath). The test file is probably
small enough on some operating systems to be sent immediately, without
waiting for the receiving side to do anything.

The fix was figured out by addaleax.

Co-authored-by: Anna Henningsen <anna@addaleax.net>
Ref: #34389 (comment)
Fixes: #34389

PR-URL: #34415
Refs: #34389
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/pummel/test-heapdump-http2.js

@@ -14,7 +14,7 @@ const http2 = require('http2');
 
 const server = http2.createServer();
 server.on('stream', (stream) => {
-  stream.respondWithFile(__filename);
+  stream.respondWithFile(process.execPath);
 });
 server.listen(0, () => {
   const client = http2.connect(`http://localhost:${server.address().port}`);