Tests for CSV parsing edge cases

[grugnog]

User Story

As an editor I want valid CSVs to be imported correctly

Acceptance Criteria

We have experienced a number of edge cases in CSV parsing, both with the default importer and the MySQL importer, including with standard files that pass CSV linting fine. We should look through these and produce some simplified test cases for each

https://github.com/maxogden/csv-spectrum also has some test cases we should incorporate. We could also check test suites for major CSV parser libraries to look for other samples and interesting edge cases.