[qa] Fix sync_blocks timeout argument

Motivation for this change is mainly to make sync_blocks behavior easier to
understand. Behavior is unchanged in the normal case when there are only 2
nodes in the rpc_connections set. When there are more than 2 nodes, the
previous "timeout -= wait" statement wouldn't take into account time spent
waiting for all nodes and as a result could lead to blocking for longer than
the requested timeout.

qa/rpc-tests/test_framework/util.py

@@ -123,10 +123,15 @@ def str_to_b64str(string):
 
 def sync_blocks(rpc_connections, *, wait=1, timeout=60):
     """
-    Wait until everybody has the same tip
+    Wait until everybody has the same tip.
+
+    sync_blocks needs to be called with an rpc_connections set that has least
+    one node already synced to the latest, stable tip, otherwise there's a
+    chance it might return before all nodes are stably synced.
     """
     maxheight = 0
-    while timeout > 0:
+    start_time = cur_time = time.time()
+    while cur_time <= start_time + timeout:
         tips = [r.waitforblockheight(maxheight, int(wait * 1000)) for r in rpc_connections]
         heights = [t["height"] for t in tips]
         if tips == [tips[0]] * len(tips):
@@ -134,8 +139,8 @@ def sync_blocks(rpc_connections, *, wait=1, timeout=60):
         if heights == [heights[0]] * len(heights):
             raise AssertionError("Block sync failed, mismatched block hashes:{}".format(
                                  "".join("\n  {!r}".format(tip) for tip in tips)))
-        timeout -= wait
         maxheight = max(heights)
+        cur_time = time.time()
     raise AssertionError("Block sync to height {} timed out:{}".format(
                          maxheight, "".join("\n  {!r}".format(tip) for tip in tips)))