test: guarantee proper lifetime for temporary Rules object: (#4917)

* Commit 01c37fe introduced a change to the STTx unit test where a local
  "defaultRules" object was created with a temporary inline "presets"
  value provided to the ctor. Rules::Impl stores a const ref to the
  presets provided to the ctor.  This particular call provided an inline
  temp variable, which goes out of scope as soon as the object is
  created. On Windows, attempting to use the presets (e.g. via the
  enabled() function) causes an access violation, which crashes the test
  run.
* An audit of the code indicates that all other instances of Rules use
  the Application's config.features list, which will have a sufficient
  lifetime.

src/test/protocol/STTx_test.cpp

@@ -1591,7 +1591,11 @@ public:
         });
         j.sign(keypair.first, keypair.second);
 
-        Rules defaultRules{{}};
+        // Rules store a reference to the presets. Create a local to guarantee
+        // proper lifetime.
+        std::unordered_set<uint256, beast::uhash<>> const presets;
+        Rules const defaultRules{presets};
+        BEAST_EXPECT(!defaultRules.enabled(featureExpandedSignerList));
 
         unexpected(
             !j.checkSign(STTx::RequireFullyCanonicalSig::yes, defaultRules),