Fix #102019 by not asking strace to follow children forks during tests.
* bzrlib/tests/test_strace.py: (TestStrace.test_strace_callable_is_called, TestStrace.test_strace_callable_result, TestStrace.test_strace_result_has_raw_log): Don't follow childrens when calling strace to avoid random hanging.
* bzrlib/strace.py: (strace): Provides a way do disable the strace '-f' option.