find or fetch catch2, use slim headers

[Krzmbrzl]

@evaleev this breaks with the find-or-fetch policy. On my system I have Catch2 installed, but I don't have the catch_test_macros.hpp headers available. I only have the combined catch.hpp header.

See also https://github.com/catchorg/Catch2/issues/2153

So it seems that we either have to

• Always fetch Catch2 and use that
• Go back to including the main catch.hpp header
• Perform explicit checks to see if the catch_test_macros exists and if it doesn't fall back to fetching Catch2 after all

EDIT: Actually, I think these headers are only part of Catch 2 v3. Thus, if we are using them, we have to explicitly require v3 or higher.