It looks like a bunch of the assertions authored in SERVER-73317 are not passing on the v6.0 branch because there is some odd '0'-suffix and an extra array. Compare these two for an example:

// Expected:
{$topN: {n: {$const: "?"}, output: {HASH(output): "$HASH(output)", HASH(sortFields): ["$HASH(sortKey)"]}, sortBy: {HASH(sortKey): 1}}}
// Actual:
{$topN: {n: {$const: "?"}, output: {HASH(output): "$HASH(output)", HASH(sortFields0): "$HASH(sortKey)"}, sortBy: {HASH(sortKey): 1}}} 

Nothing really stands out to me in the history of changes to the test file - it would have to be something between SERVER-73317 redaction for $function and $accumulator f7e235f (the original commit, where we know the tests pass) and SERVER-82368 In top/bottom accumulator map sort fields to object fields not array(the last commit to this file on v6.0). SERVER-60781 might be it?  But we may have to look at the changes to the non-test file as well to try to narrow it down.