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/ReducedOffer_test.cpp

@@ -116,7 +116,7 @@ public:
                 STAmount const initialRate = Quality(newOffer).rate();
                 std::uint32_t const bobOfferSeq = env.seq(bob);
                 STAmount const bobInitialBalance = env.balance(bob);
-                STAmount const bobsFee = drops(10);
+                STAmount const bobsFee = env.current()->fees().base;
                 env(offer(bob, newOffer.in, newOffer.out, tfSell),
                     fee(bobsFee));
                 env.close();