Fix edge case fail of shared random unittest.

The test_state_update() test would fail if you run it between 23:30 and
00:00UTC in the following line because n_protocol_runs was 2:

  tt_u64_op(state->n_protocol_runs, ==, 1);

The problem is that when you launch the test at 23:30UTC (reveal phase),
sr_state_update() gets called from sr_state_init() and it will prepare
the state for the voting round at 00:00UTC (commit phase). Since we
transition from reveal to commit phase, this would trigger a phase
transition and increment the n_protocol_runs counter.

The solution is to initialize the n_protocol_runs to 0 explicitly in the
beginning of the test, as we do for n_reveal_rounds, n_commit_rounds etc.

src/test/test_shared_random.c

@@ -1193,6 +1193,7 @@ test_state_update(void *arg)
     state->valid_after = 0;
     state->n_reveal_rounds = 0;
     state->n_commit_rounds = 0;
+    state->n_protocol_runs = 0;
   }
 
   /* We need to mock for the state update function call. */