Mr. Atria,
You left out a key element from the answer you got from stack overflow.

Note that Dimitre's response used a filter expression
(path expression) [predicate]

not a simple path expression

The ()s are critical.

$test//child/ancestor-or-self::test[ last() ]

is very different from

($test//child/ancestor-or-self::test)[ last() ]

The ancestor axis is a reverse axis, but when the xpath expression is
complete, the sequence of nodes are then put in document order
regardless of what order the last axis had.

In any event, I don't understand what is wrong with simply
$test/ancestor::element()[1]

Perhaps I don't understand your question (which is simpler than the
Stack Overflow question you linked to). But if you are just looking
for the nearest ancestor element, then it seems that
$test/ancestor::element()[1] should give it to you.