Add performance verification test about flexclass HOT 3 OPEN

I don't have experience with performance benchmarking in Catch2, but I've used it before with cmake. I'll look into what people normally do in those cases.

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It looks like that comparing the performance with only Catch2 isn't possible. It does seem though that Github Actions has support for Catch2 and performance tracking. What if we use that for performance regression testing?

I managed to build the tests with the performance test, but I couldn't quite understand if we're testing it here somehow. I see that without the -r junit to show the full textual output we get the times for each of the subsections. Are we somehow verifying that?

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When you build the performance test it will run with the regular tests but no check is really being done.
I usually manually run the binary to see the output.

So it's all very brittle today. I like the idea from the link of keeping reference performance numbers and compare against that!

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