test: enable unit tests to work with variable reference fee (#5145)

Fix remaining unit tests to be able to process reference fee values other than 10.

src/test/app/MPToken_test.cpp

@@ -1864,7 +1864,9 @@ class MPToken_test : public beast::unit_test::suite
 
         Account const alice{"alice"};
 
-        Env env{*this, features};
+        auto cfg = envconfig();
+        cfg->FEES.reference_fee = 10;
+        Env env{*this, std::move(cfg), features};
         MPTTester mptAlice(env, alice);
 
         mptAlice.create();