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 https://github.com/nodejs/node/pull/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

cc @nodejs/single-executable