pkg/fs: test, bench, and drop unnecessary check in ListBuckets

There is now a simple test and a benchmark for ListBuckets. I also
dropped an unnecessary check that was simply repeated from above,
guaranteed to be true.

pkg/fs/fs-bucket.go

@@ -78,7 +78,7 @@ func (fs Filesystem) ListBuckets() ([]BucketMetadata, *probe.Error) {
 		}
 		// If directories are found with odd names, skip them.
 		dirName := strings.ToLower(file.Name())
-		if file.IsDir() && !IsValidBucketName(dirName) {
+		if !IsValidBucketName(dirName) {
 			continue
 		}
 		metadata := BucketMetadata{