Failing test for #899: Inserting prepend multiple times overwrites instead of prepending

[schneems]

#899

[edmorley]

This is currently working as designed in libcnb.rs. So this isn't a bug, but a request to change the API/design.

See:
#899 (comment)

[schneems]

I responded in that other thread, restating here: If we cannot iterate on this interface directly, we should deprecate it in favor of one with fewer footguns.

I opened up this PR #935 and then another PR with another failing test #936 and didn't notice that I made another mistake: forgetting to add a delimiter. I just fixed it in: 7a5ff48. There is an RFC to change the upstream default buildpacks/rfcs#326, however, I don't think we need to wait for it. I think we should prefer "correctness" of programmer intent and productivity over "correctness of spec" unless doing so would violate major expectations or cause clear "one way door" type pain.