test: increase coverage of _http_outgoing

validateHeader: https://github.com/nodejs/node/blob/master/lib/_http_outgoing.js#L376
write: https://github.com/nodejs/node/blob/master/lib/_http_outgoing.js#L477
addTrailers: https://github.com/nodejs/node/blob/master/lib/_http_outgoing.js#L557

PR-URL: https://github.com/nodejs/node/pull/10820


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>