The previous commit placed the test in a new top-level tests/ directory
that does not exist in the RT-Thread repo structure and used the external
check library. Move the test to examples/utest/testcases/lwp/ and rewrite
it using RT-Thread's native utest framework, add a Kconfig symbol
(RT_UTEST_LWP_TTY_PTMX), and wire it into Kconfig.utestcases.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>