test: add postject to fixtures

This should make it possible to test out the creation of Single
Executable Applications on a PR without making outbound requests to
download and run postject using npm.

This is needed for #45038.

Refs: https://github.com/nodejs/single-executable/blob/1840f3d9c5f4fa0d29aabd5618c4ff9745f7be87/meetings/2022-10-31.md?plain=1#L75-L76
Signed-off-by: Darshan Sen <raisinten@gmail.com>

Author: RaisinTen