Add missing test for aged brie

Signed-off-by: Igor Popov <igor.popov@napa.fi>
Ignore coverage data

Signed-off-by: Igor Popov <igor.popov@napa.fi>
Add OpenCover for getting the coverage data

Signed-off-by: Igor Popov <igor.popov@napa.fi>
Remove duplicate code from tests

Signed-off-by: Igor Popov <igor.popov@napa.fi>
Remove duplicate test code for normal items

Signed-off-by: Igor Popov <igor.popov@napa.fi>
Make the code similar for sulfuras

Signed-off-by: Igor Popov <igor.popov@napa.fi>