Without command `catch_discover_tests` is not defined when not using
vendored Catch2.

Documentation suggesting we should have that line:
  * (devel) https://github.com/catchorg/Catch2/blob/devel/docs/cmake-integration.md#automatic-test-registration
  * (lastest) https://github.com/catchorg/Catch2/blob/v2.13.10/docs/cmake-integration.md#automatic-test-registration
  * (vendored) https://github.com/catchorg/Catch2/blob/v2.13.6/docs/cmake-integration.md#automatic-test-registration

It also suggest that if Catch isn't located in a standard location one should
edit `CMAKE_MODULE_PATH`. However, doing so in `extlib/CMakeLists.txt` wont
propagate the change to `tests/CMakeLists.txt`. Therefore, we conditionally
include Catch instead.