Replace mock arg with SEPA PaymentMethod instance

Test AccountAgeWitnessServiceTest > testArbitratorSignWitness failed
in full gradle build, but passed when run as a single test:

	./gradlew :core:cleanTest :core:test \
		--tests "bisq.core.account.witness.AccountAgeWitnessServiceTest"

This test also passed when run in the IDE.

Solved by not passing a mocked PaymentMethod.SEPA argument into the
test's service.getTraderPaymentAccounts() method, where mock paymentId
field was null when running full build's test suites.

Fix for bisq-network#4158

core/src/test/java/bisq/core/account/witness/AccountAgeWitnessServiceTest.java

@@ -62,6 +62,7 @@
 import org.junit.Ignore;
 import org.junit.Test;
 
+import static bisq.core.payment.payload.PaymentMethod.getPaymentMethodById;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
@@ -216,8 +217,8 @@ public void testArbitratorSignWitness() throws IOException {
         when(contract.getBuyerPaymentAccountPayload()).thenReturn(buyerPaymentAccountPayload);
         when(contract.getSellerPaymentAccountPayload()).thenReturn(sellerPaymentAccountPayload);
         when(contract.getOfferPayload()).thenReturn(mock(OfferPayload.class));
-        List<TraderDataItem> items = service.getTraderPaymentAccounts(now, PaymentMethod.SEPA, disputes);
-        assertEquals(items.size(), 2);
+        List<TraderDataItem> items = service.getTraderPaymentAccounts(now, getPaymentMethodById(PaymentMethod.SEPA_ID), disputes);
+        assertEquals(2, items.size());
 
         // Setup a mocked arbitrator key
         ECKey arbitratorKey = mock(ECKey.class);