backpatch tests: Rename conflicting role names to 14/15

Hi,

On 15 STABLE, I encountered the same issue [0] which was fixed in 6a20b04f0408
starting from 16. It seems like a decision not to backpatch to lower
versions at the
time was due to a lack of reports [1], but I think since we have to
support 14 and 15 for
a few more years, it makes sense to. I can prepare the patches if there are no
objections.

[0] https://www.postgresql.org/message-id/flat/ZvTneMVPdsMYwTQE%40paquier.xyz#162ef382dde245fd903464e66b37809b
[1] https://postgr.es/m/20221003234111.4ob7yph6r4g4ywhu@awork3.anarazel.de

--
Sami Imseih
Amazon Web Services (AWS)