chore(*): make sure LEAN_THREAD_UNSAFE build flag is handled correctly

When LEAN_THREAD_UNSAFE=ON, we:

- Do not run tests at tests/lua/threads
- Disable thread object at Lua API
- par tactical becomes an alias for interleave
- Disable some unit tests that use threads

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(*): make sure LEAN_THREAD_UNSAFE build flag is handled correctly