Unit tests don't enable Tickets or SHAMapV2 by default:

Both Tickets and SHAMapV2 have been around for a while and don't
look like they will be enabled on the network soon.  So they are
removed from the supportedAmendments list.  This prevents Env
from automatically testing with Tickets or SHAMapV2 enabled,
although testing with those features can still be explicitly
specified.

Drive-by cleanups:

o supportedAmendments() returns a const reference rather than
  a fresh vector on each call.

o supportedAmendments() implementation moved from Amendments.cpp
  to Feature.cpp.  Amendments.cpp deleted.

o supportedAmendments() declared in Feature.h.  All other
  declarations deleted.

o preEnabledAmendments() removed, since it was empty and only
  used in one place.  It will be easy to re-add when it is needed.

o jtx::all_features_except() renamed to
  jtx::supported_features_except(), which is more descriptive.

o jtx::all_amendments() renamed to jxt::supported_amendments()

o jtx::with_features() renamed to with_only_features()

o Env_test.cpp adjusted since featureTickets is no longer
  automatically enabled for unit tests.

src/test/rpc/Feature_test.cpp

@@ -116,7 +116,7 @@ class Feature_test : public beast::unit_test::suite
 
         using namespace test::jtx;
         Env env {*this,
-            with_features(featureEscrow, featureCryptoConditions)};
+            with_only_features(featureEscrow, featureCryptoConditions)};
         // The amendment table has to be modified
         // since that is what feature RPC actually checks
         env.app().getAmendmentTable().enable(featureEscrow);
@@ -220,7 +220,7 @@ class Feature_test : public beast::unit_test::suite
 
         using namespace test::jtx;
         Env env {*this,
-            with_features(featureCryptoConditions)};
+            with_only_features(featureCryptoConditions)};
         // The amendment table has to be modified
         // since that is what feature RPC actually checks
         env.app().getAmendmentTable().enable(featureCryptoConditions);