Support multiple junit-platform.properties

[holly-cummins]

Ingest properties from all junit-platform.properties files on the classpath, rather than just the first. Where the same property exists in multiple files, maintain the current behaviour.

Resolves #2794

This ended up being a fairly straightforward change, because all the testing was already in place, and just needed a slight extension for the new behaviour. What I've done is loaded all properties files with the expected name. I reverse the order so that if the same property is present in multiple files, the first occurrence 'wins'.

I also kept the warning (but with amended wording, of course). If we wanted to be really fancy we could only warn if there actually is a conflict, but I wasn't sure if the extra effort was warranted.

I strengthened the assertions in the test to equals rather than contains, since it's easy to accidentally use similar values in test properties files.

I looked through the docs but couldn't find anywhere where it says you can only have one properties file, so I don't think there's anything to change there.

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