Improve mc_report_undefined_behavior_on_fail with correct PENDING TRANSITIONS

[gc00]

The first commit is a test program that will trigger mc_report_undefined_behavior_on_fail for MCCondEnqueue. When McMini is run on this example, the "PENDING TRANSITIONS" is not correct. Hence, the user would be confused on what was the error.

The second commit fixes the logic in MCCondEnqueue.cpp to correctly display the PENDING TRANSITIONS. The commit message explains the bug and the fix:

MCCondEnqueue: mc_report_...: bad next transitions
 * Before calling mc_report_undefined_behavior_on_fail(),
   we need to call MCStack::setNextTranstion(), or else the
   "PENDING TRANSTIONS" displayed to the user will be incorrect.
 * While this fixes MCCondEnqueue, we will need to apply the same
   logic to the other transitions.
 * For testing, try condvar-two-mutexes.c, which has a bug in which
   the associated mutex of a condition variable is different from
   the mutex argument of pthread_cond_wait().

This is a draft, since we will need to apply the same fix to the logic of all of the transitions. (And perhaps we can add checks for correct usage of pthread_mutex_lock/unlock, while we're at it.