Apparently it's by design. Each individual test writes to the TestResult table, having cleared that table first. So if you run 49 tests (as you do if you run the entire SQLCop class) then you just get back the result of the last test executed from that suite.

I'm not sure I see the point in that.

There is a suggestions site within Redgate (UserVoice) where I can post a suggestion, but I must say - anybody from Redgate reading this - that is one of the ugliest web sites I have ever seen. I've tried it in IE and Chrome and it isn't pretty.