ci: Run CMake testrunner from both repo root and build dir

This updates the GitHub Actions workflow to execute `testrunner` twice,
but only for the CMake build case:
- Once from the repository root (default case).
- Once from within the `cmake.output` build directory to ensure the
  `SIMPLECPP_TEST_SOURCE_DIR` definition is correctly set and resolved.

Note that this is not yet supported for the Makefile-based build.

Suggested-by: Oliver Stöneberg <firewave@users.noreply.github.com>

Author: jcfr

.github/workflows/CI-unixish.yml

@@ -73,6 +73,9 @@ jobs:
       run: |
         cmake --build cmake.output --target testrunner -- -j $(nproc)
         ./cmake.output/testrunner
+        # Re-run tests from within the build directory to validate that
+        # SIMPLECPP_TEST_SOURCE_DIR is correctly defined and resolved
+        (cd cmake.output && ./testrunner)
 
     - name: Run valgrind
       if: matrix.os == 'ubuntu-24.04'