The test case makes use of the fine DEBUG_SYNC facility. Furthermore,
since it needs synchronization on internal threads (dump and SQL
threads) the server code has DEBUG_SYNC commands internally deployed
and activated through the DBUG_EXECUTE_IF macro. The internal
DBUG_SYNC commands are then controlled from the test case through the
DEBUG variable.

There were three problems around the DEBUG + DEBUG_SYNC facility
usage:

1. When signaling the SQL thread to continue, the test would reset
   immediately the DEBUG_SYNC variable. This could mean that the SQL
   thread might loose the signal and continue to wait forever;

2. A similar scenario was happening with the dump thread on the
   master. This thread was instructed to wait, and later it would be
   signaled to continue, but immediately after the DEBUG_SYNC would be
   reset. This could lead to the dump thread missing the signal and
   wait forever;

3. The test was not cleaning itself up with respect to the
   instrumentation of the dump thread. This would leave the
   conditional execution of an internal DEBUG_SYNC command active
   (through the usage of DBUG_EXECUTE_IF). 

We fix #1 and #2 by waiting for the threads to receive the signal and
only then issue the reset. We fix #3 by reseting the DEBUG variable,
thus deactivating the dump thread internal DEBUG_SYNC command.