tests/lapi: fix utf8_offset_test

The function `utf8.offset()` raises, the patch wraps it by `xpcall()`
and suppress errors with message "initial position is a
continuation byte".

Follows up #143

Author: ligurio

tests/lapi/utf8_offset_test.lua

@@ -19,14 +19,20 @@ if test_lib.lua_current_version_lt_than(5, 3) then
     os.exit()
 end
 
+local ignored_msgs = {
+    "initial position is a continuation byte",
+}
+
 local function TestOneInput(buf)
     local fdp = luzer.FuzzedDataProvider(buf)
     local max_len = fdp:consume_integer(0, MAX_INT)
     local s = fdp:consume_string(max_len)
     local n = fdp:consume_integer(MIN_INT, MAX_INT)
     local i = fdp:consume_integer(1, MAX_INT)
     os.setlocale(test_lib.random_locale(fdp), "all")
-    utf8.offset(s, n, i)
+    local err_handler = test_lib.err_handler(ignored_msgs)
+    local ok, _ = xpcall(utf8.offset, err_handler, s, n, i)
+    if not ok then return end
 end
 
 local args = {