tl::expected didn't respect LIBCORO_BUILD_TESTS=OFF

[jbaldwin]

Closes #143

[dok-net]

OK, got it, verifies here. The magic is in the order of merging the lines in my test. If CMakeLists.txt has it like this, snip snip:

if(NOT LIBCORO_EXTERNAL_DEPENDENCIES)
    if(NOT LIBCORO_BUILD_TESTS)
        # Disable testing in expected
        set(BUILD_TESTING OFF CACHE INTERNAL "")
    endif()
    add_subdirectory(vendor/tartanllama/expected)
else()
    find_package(tl-expected CONFIG REQUIRED)
endif()

i.e. fist disable, then add subdirectory, the lenghty test builds are omitted.

Merge away :-)

[jbaldwin]

Woohoo!