Before there were only two: "main" and "idle". But now there can also be
a thread for "usbus". The code will now use sched_num_threads and it
subtracts that number from MAXTHREADS.
This resolves issue #14180
- Define test_utils_interactive_sync as DEFAULT_MODULE in Makefile.tests_common
- For tests disabling autoinit, add test_utils_interactive_sync to main
- Add DISABLE_MODULE += test_utils_interactive_sync for tests requiring
sudo, `tests/shell`, `tests/minimal` and `tests/stdin`
- Add shell_commands to tests/periph_wdt and tests/struct_tm_utility to
pull `r` and `s` commands
- Remove includes and usage in `tests/main.c` for tests that where
already using test_utils_interactive_sync
