7852: add a failing test that does: readline and then readlines. The expectation...

7852: add a failing test that does: readline and then readlines. The expectation would be the second call,
readlines, would fetch all the remaining lines after the first line. However, the readlines call is only
fetching the data after the "first block" which was read during the first readline call. It appears that
self._readline_cache and self._filepos are playing a role here, which are set during the first readline.