Skip unit test builds for simlibs_debug and simlibs_release

[jonyMarino]

Fixes: #

About

This PR updates the simlibs build flow so simlibs_debug and simlibs_release only build/package the sim libraries and no longer build unit test executables as part of the default CMake build target.

The unit test executables are now marked with EXCLUDE_FROM_ALL, which keeps them out of the default cmake --build invocation used by simlibs_debug and simlibs_release.

The explicit test targets still work:

• test_simlibs_debug
• test_simlibs_release

Those targets now build the new simlibs_unit_tests target before running ctest, preserving the existing test workflow when tests are requested directly.

How Has This Been Tested?

• Verified the CMake/test target changes statically.
• Verified all simlibs unit test executables are excluded from the default build target.
• Ran git diff --check successfully.

Screenshots and videos (if appropriate):

N/A