test: Fix sign for expected values

A number of BOOST_CHECK_EQUAL calls would result in warnings about signs.

src/test/main_tests.cpp

@@ -55,7 +55,7 @@ BOOST_AUTO_TEST_CASE(subsidy_limit_test)
         nSum += nSubsidy * 1000;
         BOOST_CHECK(MoneyRange(nSum));
     }
-    BOOST_CHECK_EQUAL(nSum, 2099999997690000ULL);
+    BOOST_CHECK_EQUAL(nSum, CAmount{2099999997690000});
 }
 
 bool ReturnFalse() { return false; }