Enhancement: Add tests for new Select Backlog columns feature

[peterVG]

This feature will be available in release 1.12. See archivematica/Issues#1167

Feature: Select which columns to display in the table list on the Backlog tab
Scenario: The user has moved transfer packages to Backlog and is now ready to take further actions on them.
Given: That they can browse packages using the table list, column sorting, and list paging on the Backlog tab.
And: The default Name, Transfer UUID, File Count, Ingest Date, and Actions columns are currently displayed in the table list.
When: They click on the column selector icon, they can deselect one or more of the currently displayed default columns and can select one or more options in their place.
Then: The columns displayed in the table list on the Backlog tab match the selected and deselected options.
And: These selections will also apply to the “Show files” display option (except “File Count”)
And: These selections will be applied to all users of the pipeline
And: The select and deselect choices will persist between user sessions

[sallain]

@peterVG If you could add a test for this before the end of August that would be great!

[peterVG]

Done. PR submittted. See 523bf20 and also 763260f for archival storage tab columns.