colblk: fix for BenchmarkUnsafeIntegerSlice

Take the random row index generation out of the measured path.

```
name                                 old time/op  new time/op  delta
UnsafeIntegerSlice/uint64,delta8-10  3.54ns ± 0%  1.25ns ± 1%  -64.65%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint64,delta4-10  3.44ns ± 0%  0.94ns ± 0%  -72.70%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint64,delta2-10  3.36ns ± 0%  0.63ns ± 0%  -81.36%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint64,delta1-10  3.30ns ± 0%  0.63ns ± 1%  -80.74%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint64,delta0-10  3.30ns ± 1%  0.51ns ± 0%  -84.43%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint32,delta4-10  3.44ns ± 0%  0.94ns ± 2%  -72.55%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint32,delta2-10  3.36ns ± 0%  0.63ns ± 1%  -81.39%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint32,delta1-10  3.34ns ± 3%  0.62ns ± 0%  -81.38%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint32,delta0-10  3.33ns ± 2%  0.51ns ± 0%  -84.80%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint16,delta2-10  3.40ns ± 1%  0.64ns ± 5%  -81.29%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint16,delta1-10  3.30ns ± 0%  0.62ns ± 0%  -81.13%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint16,delta0-10  3.29ns ± 0%  0.51ns ± 0%  -84.63%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint8,delta1-10   3.29ns ± 0%  0.62ns ± 0%  -81.09%  (p=0.029 n=4+4)
UnsafeIntegerSlice/uint8,delta0-10   3.30ns ± 0%  0.51ns ± 0%  -84.65%  (p=0.029 n=4+4)
```

sstable/colblk/unsafe_slice_test.go

@@ -6,6 +6,7 @@ package colblk
 
 import (
 	"fmt"
+	"io"
 	"math"
 	"testing"
 	"time"
@@ -46,9 +47,16 @@ func benchmarkUnsafeIntegerSlice[U Uint](
 		buf := aligned.ByteSlice(int(sz) + 1 /* trailing padding byte */)
 		_ = ub.Finish(0, rows, 0, buf)
 		s, _ := DecodeUnsafeIntegerSlice[U](buf, 0, rows)
+		var reads [256]int
+		for i := range reads {
+			reads[i] = rng.Intn(rows)
+		}
 		b.ResetTimer()
+		var result uint8
 		for i := 0; i < b.N; i++ {
-			_ = s.At(rng.Intn(rows))
+			result ^= uint8(s.At(reads[i&255]))
 		}
+		b.StopTimer()
+		fmt.Fprint(io.Discard, result)
 	})
 }