[Feature Request] clean up io tests and plan a new set

[Freymaurer]

Is your feature request related to a problem? Please describe.

Currently IO tests were created ad-hoc and not really planned. These should get cleaned up and planned out in a senseful way.

Describe the solution you'd like

We must cover:

• Reading/Writing to/from different io libraries
• Tables

So for FsSpreadsheet.ExcelIO we must test:

• READ fom exceljs
• READ from excel (the app)
• READ from ClosedXML
• READ from libre
• READ from FsSpreadsheet
• ROUNDABOUT FsSpreadsheet.ExcelIO
• WRITE

ForFsSpreadsheet.Exceljs we must test:

• READ fom FsSpreadsheet.ExcelIO
• READ from excel (the app)
• READ from ClosedXML #77
• READ from libre
• READ from FsSpreadsheet
• ROUNDABOUT FsSpreadsheet.Exceljs
• WRITE

More

• All files must contain a table with different cell value types. We propose the following:

Numbers	Strings	DateTime	Boolean	ARCtrl Column	ARCtrl Column
1	Hello	any	true	(A) This is part 1 of 2	(A) This is part 2 of 2
2	World	any	false		Tests if column names with whitespace at end can be unique
3	Bye	any	true
4	Outer Space	any	false

• Duplicate this Table to TWO worksheets to also include multiple worksheets.

• Due to the nature of these tests, we can recycle most of the test files. We only need one file of any kind for read-in.

• All files written by tests should be prefixed with "WRITE_" so they can be deleted at any point, while all other files MUST never be opened AND saved to avoid corrupting their origin.