configure finds Catch2 and then still complains about the missing catch submodule; tests are broken

[yurivict]

-- Found Catch2: /usr/local/lib/cmake/Catch2 (version 2.13.9)
CMake Error at CMakeLists.txt:296 (add_subdirectory):
  The source directory

    /usr/ports/science/openmc/work/openmc-0.13.3/vendor/Catch2

  does not contain a CMakeLists.txt file.

This comparison should probably be if (NOT Catch2_FOUND).

Once this is corrected - it still fails:

-- Configuring done
CMake Error at tests/cpp_unit_tests/CMakeLists.txt:8 (target_link_libraries):
  Target "test_distribution" links to:

    Catch2::Catch2WithMain

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

And when the Catch2 submodule is used - the latter error still happens.
So tests are currently broken.

Version: 0.13.3
catch-2.13.9
FreeBSD 13.1