Testsuite TxUtil class contain superfluous method enlistTestXAResource

With fix for WFLY-7205

there was introduced ambiguousness on method enlistTestXAResource in
https://github.com/wildfly/wildfly/blob/master/testsuite/shared/src/main/java/org/jboss/as/test/integration/transactions/TxTestUtil.java

This needs to be eliminated and only one such method should exist.