Add more unit-tests of cross-band source association

[timstaley]

cf https://redmine.astron.nl/lofar_issuetracker/issues/3721

Although source association is well-tested for single-band data, we could use some quality assurance on multi-band datasets (although no problems encountered in real-world usage yet). As a bare minimum we should ensure all the test-cases outlined in the docs have accompanying tests.

[AntoniaR]

Unclear if this is still an issue so would be good to check if the latest version has these tests and if they are useful.