Rearrange _thread_test_func to please Coverity Scan

I believe  that since  we were allocating  *cp while holding  a mutex,
coverity deduced that  *cp must be protected by  that mutex, and later
flipped out  when we didn't  use it  that way. If  this is so,  we can
solve our problems by moving the *cp = tor_strdup(buf) part outside of
the mutex-protected code.