Package testing define helper / mock for unit testing
Because golang likes return error object instead of exception/panic,
always handle error return values is a good practise. But sometimes it is
impossible to got error, such as read from memory buffer, not handler them
maybe maybe loose error because someday code changes, but handle them needs
a lot of duplicate codes.
In package testing contains many test helper packages, suffix with `th', to
handle these never happen errors. Test helper check the error result, if it
is not nil, using testing.Fatal(err) to log the error object and abort current
test case execution.