Minimal C changes to implement virtual root support

Includes the minimal changes to get the Python tests passing as well.

Skipping viz tests for now, as the rendering is arbitrary.

Closes #1670

Author: jeromekelleher

c/tests/test_trees.c

@@ -1338,7 +1338,9 @@ test_simplest_multi_root_tree(void)
     CU_ASSERT_EQUAL(tsk_treeseq_get_num_trees(&ts), 1);
 
     ret = tsk_tree_init(&t, &ts, 0);
+
     tsk_tree_print_state(&t, _devnull);
+
     /* Make sure the initial roots are set correctly */
     CU_ASSERT_EQUAL(t.left_root, 0);
     CU_ASSERT_EQUAL(t.left_sib[0], TSK_NULL);
@@ -1348,7 +1350,6 @@ test_simplest_multi_root_tree(void)
     CU_ASSERT_EQUAL(t.left_sib[2], 1);
     CU_ASSERT_EQUAL(t.right_sib[2], TSK_NULL);
 
-    CU_ASSERT_EQUAL(ret, 0);
     CU_ASSERT_EQUAL(ret, 0);
     ret = tsk_tree_first(&t);
     CU_ASSERT_EQUAL(ret, 1);