types: use proper lowercase syntax for coins in benchmarks (#8435)

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

Fixes #8433

types/coin_benchmark_test.go

@@ -6,7 +6,7 @@ import (
 )
 
 func coinName(suffix int) string {
- return fmt.Sprintf("COINZ_%d", suffix)
+ return fmt.Sprintf("coinz%d", suffix)
 }
 
 func BenchmarkCoinsAdditionIntersect(b *testing.B) {