Clarify Test property from Internationalization

[programneer]

Since Test property was also used in non-editor builds, i think an better solution would be to rename it to Override.

[akien-mga]

That makes sense to me, but this breaks compatibility, so it's tricky.

We should also consider whether the bug here is the documentation being inaccurate, or the fact that something apparently intended to be an editor-only test option, also works in exported builds.

For editor-only testing, we ought to have better ways to expose this kind of feature than forcing users to modify the project settings, like the new "Customize Run Instances" dialog in 4.3:

This already allows overriding the locale by passing --lang fr as argument I believe, but it can maybe be made more user-friendly.

To be clear I'm not saying the compat breakage of renaming this setting wouldn't be acceptable. We just need to make sure we consider the problem fully and provide the solution(s) that are needed. I think renaming this only handles part of the issue.