Use features instead of ApplyFlags:

tapENABLE_TESTING is removed from checks, and feature enablement
is the sole method for activating features. Unit tests are updated
to enable required features in the construction of the Env.

Tickets are put on a feature switch instead of a build macro.

src/ripple/app/tx/impl/SetSignerList.cpp

@@ -73,9 +73,8 @@ SetSignerList::determineOperation(STTx const& tx,
 TER
 SetSignerList::preflight (PreflightContext const& ctx)
 {
-    if (! (ctx.flags & tapENABLE_TESTING) &&
-            ! ctx.rules.enabled(featureMultiSign,
-                ctx.app.config().features))
+    if (! ctx.rules.enabled(featureMultiSign,
+            ctx.app.config().features))
         return temDISABLED;
 
     auto const ret = preflight1 (ctx);