Fix WORKING_DIRECTORY for unit tests

[Bktero]

Hello

I ran into a path-related issue with ctest in a project that fetches jsoncons.

The issue can be reproduced with the following CMakeLists.txt:

cmake_minimum_required(VERSION 3.25)

project(test_jsoncons)

include(FetchContent)

FetchContent_Declare(jsoncons
        GIT_REPOSITORY https://github.com/danielaparker/jsoncons.git
        GIT_TAG v0.176.0
)

FetchContent_MakeAvailable(jsoncons)

enable_testing()

Then we can run the following commands:

% cmake -S . -B build -G Ninja
% cd build
% ninja 
% ctest  
Test project /home/pierre/temp/test_jsoncons/build
    Start 1: unit_tests
Failed to change working directory to /home/pierre/temp/test_jsoncons/test : No such file or directory
1/1 Test #1: unit_tests .......................***Not Run   0.00 sec

0% tests passed, 1 tests failed out of 1

Total Test time (real) =   0.00 sec

The following tests FAILED:
          1 - unit_tests (Not Run)
Errors while running CTest
Output from these tests are in: /home/pierre/temp/test_jsoncons/build/Testing/Temporary/LastTest.log
Use "--rerun-failed --output-on-failure" to re-run the failed cases verbosely.

Just in case, my compiler is:

% gcc --version
gcc (Debian 12.2.0-14) 12.2.0

This PR fixes this issue.

ctest still runs properly when the commands above are executed from the root of the jsoncons repository.

Note that these commands don't work when they are executed from the test directory. The first lines seems to foresee this possibilities. However, catch2 is not found when ninja runs: fatal error: catch/catch.hpp: No such file or directory. I have not attempted to address this issue, as it was present before my commit for this PR.

Best regards
Bktero