test: replace setTimeout with setImmediate in stream test

Replace setTimeout() with setImmediate() in
test-stream-writable-clear-buffer. The test still fails in Node.js 8.6.0
(if you comment out the `common` module, since it uses BigInts and those
aren't supported in 8.6.0).

PR-URL: https://github.com/nodejs/node/pull/30561


Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>