[4.2.0] tests expect test data in /usr/data/

[olebole]

When running the tests on the just-built package, many tests complain about missing test data which they expected under /usr/data/, like:

6: terminate called after throwing an instance of 'std::runtime_error'
6:   what():  Test data is missing: /usr/data/expected/operators/u_data

9: purify::critical  Missing file will be removed from list: /usr/data/atca/0332-391.uvfits

For the tests, it would be useful if the test data could be taken from the source instead of being expected to be installed. And /usr/data/ is not conform to the Linux file system standard and a bit generic for these specific data. Could you give me a hint how this can be adjusted?

For completeness, I will attached the full build (and test) log in the previous issue:
purify_4.2.0-1_amd64.build.txt