ffldb: throw error when attempting to delete an open file

This change lets us test that we don't attempt to delete open files with
unit tests.  On Windows this behavior is not allowed which results in an
error but on linux and osx it's allowed. To better test Windows
compatibilty adding this explicit check in is useful.

database/ffldb/blockio.go

@@ -307,6 +307,11 @@ func (s *blockStore) openFile(fileNum uint32) (*lockableFile, error) {
 // other state cleanup necessary.
 func (s *blockStore) deleteFile(fileNum uint32) error {
 	filePath := blockFilePath(s.basePath, fileNum)
+	blockFile := s.openBlockFiles[fileNum]
+	if blockFile != nil {
+		err := fmt.Errorf("attempted to delete open file at %v", filePath)
+		return makeDbErr(database.ErrDriverSpecific, err.Error(), err)
+	}
 	if err := os.Remove(filePath); err != nil {
 		return makeDbErr(database.ErrDriverSpecific, err.Error(), err)
 	}