[rpc-tests] Change solve() to use rehash

Github-Pull: #7468
Rebased-From: 7689041

Author: mrbandrews

qa/rpc-tests/test_framework/mininode.py

@@ -536,7 +536,7 @@ def is_valid(self):
         return True
 
     def solve(self):
-        self.calc_sha256()
+        self.rehash()
         target = uint256_from_compact(self.nBits)
         while self.sha256 > target:
             self.nNonce += 1