tests: logging: restore userspace testing

Restore userspace testing that was removed with commit 'aa2b72d3
(tests: logging: remove userspace testing, 2024-04-10)' and reported in
the issue #49213.

For the tests running with threads in user space (with ZTEST_USER),
the memory partitions are configured in the test-suite setup.

Signed-off-by: Christoph Winklhofer <cj.winklhofer@gmail.com>