Ensure output directory exists

If needed, create the directory for the intended output file. This
allows outputs to be grouped into subdirectories instead of only in the
main history directory.

Author: Jonathan Van Eenwyk

openhtf/output/callbacks/__init__.py

@@ -22,6 +22,7 @@
 
 import collections
 import contextlib
+import os
 import shutil
 import tempfile
 
@@ -82,6 +83,10 @@ def serialize_test_record(test_record):
   @staticmethod
   def open_file(filename):
     """Override method to alter file open behavior or file types."""
+    basepath = os.path.dirname(filename)
+    if not os.path.exists(basepath):
+      os.makedirs(basepath)
+
     return Atomic(filename)
 
   def create_file_name(self, test_record):