Test: month column has all expected months, year column has all expected years

[newsroomdev]

Please read how to create a new test if you're interested in writing this test.

[ejfox]

@geraldarthur Can you expand on this idea a bit?

[newsroomdev]

Sure. If a column contains dates, we want to test for major gap between dates. We initially thought of this when we had to plot simple timeseries data, but were missing a whole month or year of data. Happy to accept pull requests on this. Would just want to test for the following gaps:

• [ ] year column with an ordered series of numbers skips a year or more #53
• [ ] month column with an ordered series of months skips a month or more #54

datetime tests quality for their own test suite imo