test: enable compile time param to change reference fee value (#5159)

Adds an extra CI pipeline to perform unit tests using different values for fees.

src/test/jtx/impl/envconfig.cpp

@@ -33,7 +33,7 @@ setupConfigForUnitTests(Config& cfg)
     using namespace jtx;
     // Default fees to old values, so tests don't have to worry about changes in
     // Config.h
-    cfg.FEES.reference_fee = 10;
+    cfg.FEES.reference_fee = UNIT_TEST_REFERENCE_FEE;
     cfg.FEES.account_reserve = XRP(200).value().xrp().drops();
     cfg.FEES.owner_reserve = XRP(50).value().xrp().drops();