Create test/data directory if it doesn't exist.

Several commits ago, I removed the .gitignore from the test/data
directory.  Since this was the only file in the directory, this removed
the directory from the git repository, and it wasn't present in new
clones.

This caused several data_storer tests to fail which depended on that
directory path existing.

This commit changes the data_storer tests to create the directory on
startup if it doesn't exist.