Minimize use of jtx::with_only_features (RIPD-1515):

In order to automatically run unit tests with newly created
amendments, prefer to start with jtx::supported_features() and
then subtract unwanted features.

These changes identified a few bugs that were hiding in
amendments.  One of those bugs, in FlowCross, is not yet fixed.
By uncommenting the test in CrossingLimits_test.cpp you can see
failures relating to that bug.  Since FlowCross is not yet
enabled on the network we can fix the bug at our convenience.

src/test/rpc/LedgerRPC_test.cpp

@@ -476,8 +476,7 @@ class LedgerRPC_test : public beast::unit_test::suite
                 cfg->section("transaction_queue")
                     .set("minimum_txn_in_ledger_standalone", "3");
                 return cfg;
-            }),
-            with_features(featureFeeEscalation, fix1513)};
+            })};
 
         Json::Value jv;
         jv[jss::ledger_index] = "current";