Test migration to Catch 3.x

[offa]

How about migrating tests to Catch 3.x, the latest version?

v3 is the next major version of Catch2 and brings three significant changes:

• Catch2 is now split into multiple headers
• Catch2 is now compiled as a static library
• C++14 is the minimum required C++ version

https://github.com/catchorg/Catch2/blob/devel/docs/migrate-v2-to-v3.md

Since it's no longer header-only we need some other way to consume it though. Eg. using Conan and/or as precondition?