db/state: skip Benchmark_BtreeIndex_Search when data file is absent (#19506)

`Benchmark_BtreeIndex_Search` hardcodes a path to an external data file
(`../../data/storage.256-288.kv`) that doesn't exist in the repo,
causing the benchmark to fail rather than skip when the file is absent.

This applies the same `os.Stat` + `b.Skip()` guard already used by
`Benchmark_Recsplit_Find_ExternalFile` in the same file.

Spotted while experimenting with benchmarks.

Author: anacrolix

db/state/aggregator_bench_test.go

@@ -109,11 +109,16 @@ func queueKeys(ctx context.Context, seed, ofSize uint64) <-chan []byte {
 }
 
 func Benchmark_BtreeIndex_Search(b *testing.B) {
+	dataPath := "../../data/storage.256-288.kv"
+	f, err := os.Stat(dataPath)
+	if err != nil || f.IsDir() {
+		b.Skip("requires existing KV file at ../../data/storage.256-288.kv")
+	}
+
 	logger := log.New()
 	rnd := newRnd(uint64(time.Now().UnixNano()))
 	tmp := b.TempDir()
 	defer dir.RemoveAll(tmp)
-	dataPath := "../../data/storage.256-288.kv"
 
 	indexPath := filepath.Join(tmp, filepath.Base(dataPath)+".bti")
 	comp := seg.CompressKeys | seg.CompressVals