libyang REFACTOR return values and options changes

Unified, all functions that return a variety
of errors return LY_ERR. Any options such as
NOSIBLINGS, WITHSIBLINGS and so on were replaced
by function variants with suffixes _single, _all,
_tree, and _siblings.
diff --git a/tests/utests/test_xpath.c b/tests/utests/test_xpath.c
index 32f0f8d..9301f7d 100644
--- a/tests/utests/test_xpath.c
+++ b/tests/utests/test_xpath.c
@@ -126,8 +126,8 @@
 #endif
 
     assert_int_equal(LY_SUCCESS, ly_ctx_new(TESTS_DIR_MODULES_YANG, 0, &ctx));
-    assert_non_null(lys_parse_mem(ctx, schema_a, LYS_IN_YANG));
-    assert_non_null(lys_parse_mem(ctx, schema_b, LYS_IN_YANG));
+    assert_int_equal(lys_parse_mem(ctx, schema_a, LYS_IN_YANG, NULL), LY_SUCCESS);
+    assert_int_equal(lys_parse_mem(ctx, schema_b, LYS_IN_YANG, NULL), LY_SUCCESS);
 
     return 0;
 }