This comment from the PR suggests you may have mistaken user.dir for user.home:

If the env variable is not present the test data will be assumed in a bc-test-data folder in your user root dir.

user.dir is the directory Java was launched from, not your user root dir. The search process first looks there and then back up along the path. The intended simple config is to checkout bc-test-data as a sibling of bc-java; note that the same test data is also looked for by other BC projects by the same logic (e.g. bc-csharp and the FIPS and LTS versions) - it was previously included in all those projects separately.

I think adding a comment to this effect in the Building section is fine; I'm less keen on introducing an environment variable.

Did the mistake first, realized it later but did not updated the comment ;D

I'm fine with doing only the change in the README. But that should really be done since I felt into exactly that pitfall. I will update the PR based on your feedback

I've updated the README, it's a good idea. My instinct it to go no further than that, I'll let this sit for a bit while I think on it further.