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/schema/test_schema_stmts.c b/tests/utests/schema/test_schema_stmts.c
index 9e34742..97ad697 100644
--- a/tests/utests/schema/test_schema_stmts.c
+++ b/tests/utests/schema/test_schema_stmts.c
@@ -33,7 +33,7 @@
     *state = test_identity;
 
     struct ly_ctx *ctx;
-    struct lys_module *mod, *mod_imp;
+    const struct lys_module *mod, *mod_imp;
 
     assert_int_equal(LY_SUCCESS, ly_ctx_new(NULL, 0, &ctx));
 
@@ -161,7 +161,7 @@
     *state = test_feature;
 
     struct ly_ctx *ctx;
-    struct lys_module *mod;
+    const struct lys_module *mod;
     const struct lysc_feature *f, *f1;
 
     assert_int_equal(LY_SUCCESS, ly_ctx_new(NULL, 0, &ctx));