Allow specifying a scene in completion tests

[HolonProduction]

Followup to #85178.

• Adds the option to specify a scene path in the test cfg file. This will come in handy when performing many tests against the same scene (e.g. testing everything related to get_node).
• Fixes an oversight in the original PR, which meant that scene files would not correctly work in sub folders.
• Solve inconsistencies in key naming for the cfg file. (They were different between code and example test).
• Also documents the format for completion tests inside the test README

[YuriSizov]

Thanks!