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/jtx/Env.h

@@ -151,6 +151,37 @@ supported_features_except (uint256 const& key, Args const&... args)
         std::array<uint256, 1 + sizeof...(args)>{{key, args...}});
 }
 
+/**
+ * @brief create collection of features to pass to Env ctor
+ *
+ * The resulting collection will contain *all supported amendments* plus
+ * the features passed as arguments.
+ *
+ * @param keys features to include in the resulting collection
+ */
+template<class Col>
+FeatureBitset
+supported_features_plus (Col const& keys)
+{
+    return supported_amendments() | makeFeatureBitset(keys);
+}
+
+/**
+ *
+ * @brief create collection of features to pass to Env ctor
+ * The resulting collection will contain *all supported amendments* plus
+ * the features passed as arguments.
+ *
+ * @param key+args features to include in the resulting collection
+ */
+template <class... Args>
+FeatureBitset
+supported_features_plus (uint256 const& key, Args const&... args)
+{
+    return supported_features_plus(
+        std::array<uint256, 1 + sizeof...(args)>{{key, args...}});
+}
+
 class SuiteSink : public beast::Journal::Sink
 {
     std::string partition_;

src/test/jtx/Env_test.cpp

@@ -355,7 +355,7 @@ public:
     {
         using namespace jtx;
 
-        Env env(*this, with_only_features(featureMultiSign));
+        Env env(*this);
         env.fund(XRP(10000), "alice");
         env(signers("alice", 1,
             { { "alice", 1 }, { "bob", 2 } }),                  ter(temBAD_SIGNER));
@@ -384,7 +384,7 @@ public:
         ticket::create("alice", 60, "bob");
 
         {
-            Env env(*this, with_only_features(featureTickets));
+            Env env(*this, supported_features_plus (featureTickets));
             env.fund(XRP(10000), "alice");
             env(noop("alice"),                  require(owners("alice", 0), tickets("alice", 0)));
             env(ticket::create("alice"),        require(owners("alice", 1), tickets("alice", 1)));