Bug#40990 Maria: failure of maria.test & maria_notemebedded in deadlock detection

detect a case when a blocker has removed itself and signalled after the condition timed out
but before it (cond_wait) acquired the mutex back

include/waiting_threads.h

@@ -185,6 +185,7 @@ typedef struct st_wt_thd {
 #define WT_OK                   0
 #define WT_DEADLOCK             -1
 #define WT_DEPTH_EXCEEDED       -2
+#define WT_FREE_TO_GO           -3
 
 void wt_init(void);
 void wt_end(void);