CMake tests ignore installed Catch2

[mpoquet]

As of latest commit 3ce61a1 or latest release v2.1.1, CMake tests always download Catch2 from a GitHub release instead of using an installed version if available.

This forces me to disable tests in the CLI11 Nix package in nixpkgs (I am updating it to CLI-v2.1.1) since Nix builds packages in a sandbox where they can only access their sources and not download external resources.

Can you update Catch2 CMake integration so that an installed Catch2 version is used if available? Installed Catch2 seems to be retrievable from CMake either via pkg-config (catch2.pc) or via CMake itself. This would change #include "catch.hpp" in tests to #include <catch2/catch.hpp>.