tests/subsys/settings: build common code as zephyr_libraries

Stops leaking very long source paths in build directories; makes them
deterministic. Finishes what was started in commit b4282bf7, see
rationale and code reviews there.

See also CMake issue https://gitlab.kitware.com/cmake/cmake/issues/19475


for more details.

Use the opportunity to remove the most obvious duplication.

Test with: sanitycheck -T $ZEPHYR_BASE/tests/subsys/settings/

Signed-off-by: Marc Herbert <marc.herbert@intel.com>