@Test
    public void shouldHandleTxCommit() {
        try {
            eval("g.tx().commit()");
            fail("Should not have succeeded because EmptyGraph does not support transactions.");
        } catch(UnsupportedOperationException uoe) {
            assertEquals("Graph does not support transactions", uoe.getMessage());
        }
    }
Can only test against {@link EmptyGraph} which won't support transactions so all it can verify is that {@code tx()}
is called to fail.
gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/language/grammar/TraversalSourceTxVisitorTest.java