test: add stdin writable regression test

Make sure that `process.stdin.write()`, and in particular
ending the stream, works.

Backport-PR-URL: #25351
PR-URL: #23053
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

test/pseudo-tty/test-stdin-write.js

@@ -0,0 +1,3 @@
+'use strict';
+require('../common');
+process.stdin.end('foobar\n');

test/pseudo-tty/test-stdin-write.out

@@ -0,0 +1 @@
+foobar