types: use proper lowercase syntax for coins in benchmarks

[odeke-em]

Instead of
COINZ_%d
use
coinZ%d
which is the syntax accepted by types.ParseCoin, to generate
coins to use in benchmarking.

Fixes #8433

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[amaury1093]

sgtm

[odeke-em]

Thank you for the review and approval @amaurymartiny!
I could kindly use another blocking approval as this change is simple and blocks my background work, thanks!