Fix wrong ibid position, test transparent locators - #399

According to the CSL spec¹ (emphasis mine):

The presence of locators determines which position is assigned:

• Preceding cite does not have a locator: if the current cite has a locator, the position of the current cite is “ibid-with-locator”. Otherwise the position is “ibid”.
• Preceding cite does have a locator: if the current cite has the same locator, the position of the current cite is “ibid”. If the locator differs the position is “ibid-with-locator”. If the current cite lacks a locator its only position is “subsequent”.

Prior logic only checked if the locators differed in any way, assigning "ibid with locator" if so. But that included differing by the previous citation having a locator and the current one not having one, whereas that case should be handled differently per the spec, and the position should be subsequent instead.
(Related: #301, though that fixed a different bug.)

Added tests for transparent locators, which not only verify this behavior has been fixed but also properly test the additions in #299.