test: remove invalid test

`test/known_issues/test-stdout-buffer-flush-on-exit.js` is invalid. The
behavior seen currently (with the test failing) is in accordance with
documentation which indicates that calling `process.exit()` may mean
scheduled asynchronous I/O does not happen. The documentation also
indicates that `process.stdout` is asynchronous when it is a POSIX pipe.

PR-URL: https://github.com/nodejs/node/pull/15320
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/known_issues/test-stdout-buffer-flush-on-exit.js

@@ -1,26 +0,0 @@
-'use strict';
-// Refs: https://github.com/nodejs/node/issues/2148
-
-require('../common');
-const assert = require('assert');
-const execSync = require('child_process').execSync;
-
-const lineSeed = 'foo bar baz quux quuz aaa bbb ccc';
-
-if (process.argv[2] === 'child') {
-  const longLine = lineSeed.repeat(parseInt(process.argv[4], 10));
-  process.on('exit', () => {
-    console.log(longLine);
-  });
-  process.exit();
-}
-
-[22, 21, 20, 19, 18, 17, 16, 16, 17, 18, 19, 20, 21, 22].forEach((exponent) => {
-  const bigNum = Math.pow(2, exponent);
-  const longLine = lineSeed.repeat(bigNum);
-  const cmd =
-    `"${process.execPath}" "${__filename}" child ${exponent} ${bigNum}`;
-  const stdout = execSync(cmd).toString().trim();
-
-  assert.strictEqual(stdout, longLine, `failed with exponent ${exponent}`);
-});