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: https://github.com/nodejs/node/issues/34389#issuecomment-659297173
Fixes: https://github.com/nodejs/node/issues/34389

PR-URL: https://github.com/nodejs/node/pull/34415
Refs: https://github.com/nodejs/node/issues/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>