Add a test to confirm that CDF file has the expected data in it

[ehsteve]

We were testing by file size but this put us at risk of the test failing if the CDF library made any changes to how data is stored. A test should be added which opens the CDF file and confirms that the data in it is what is expected as in it should check actual values and/or meta data. Those known values can be baked into the test or another into another file such as a text file which could be compared against.