Fix pow_tests to work with Equihash

Changing the order of difficulty calculation operations to divide first doesn't
affect the result significantly, but ensures we never overflow the arith_uint256
during multiplication and get an artificial jump in difficulty.

src/pow.cpp

@@ -71,8 +71,8 @@ unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nF
     arith_uint256 bnOld;
     bnNew.SetCompact(pindexLast->nBits);
     bnOld = bnNew;
-    bnNew *= nActualTimespan;
     bnNew /= params.nPowTargetTimespan;
+    bnNew *= nActualTimespan;
 
     if (bnNew > bnPowLimit)
         bnNew = bnPowLimit;

src/test/pow_tests.cpp

@@ -37,8 +37,9 @@ BOOST_AUTO_TEST_CASE(get_next_work_pow_limit)
     CBlockIndex pindexLast;
     pindexLast.nHeight = 2015;
     pindexLast.nTime = 1233061996;  // Block #2015
-    pindexLast.nBits = 0x1d00ffff;
-    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, params), 0x1d00ffff);
+    // TODO change once the harder genesis block is generated
+    pindexLast.nBits = 0x207fffff;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, params), 0x207fffff);
 }
 
 /* Test the constraint on the lower bound for actual time taken */
@@ -52,7 +53,7 @@ BOOST_AUTO_TEST_CASE(get_next_work_lower_limit_actual)
     pindexLast.nHeight = 68543;
     pindexLast.nTime = 1279297671;  // Block #68543
     pindexLast.nBits = 0x1c05a3f4;
-    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, params), 0x1c0168fd);
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, params), 0x1c0168fc);
 }
 
 /* Test the constraint on the upper bound for actual time taken */