Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
- 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
