test: fix test-https-writable-true-after-close

test-https-writable-true-after-close fails intermittently when run with
a lot of competing processes. Move it to sequential for stability.

PR-URL: https://github.com/nodejs/node/pull/15705
Fixes: https://github.com/nodejs/node/issues/15700


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>