add tests for Excel files with links

[gdementen]

def round_trip():
    assert read_excel('BookA.xlsx')['link to cell with link to other workbook', 'value from link'] == 4
    assert read_excel('BookB.xlsx')['cell with link to other workbook', 'formula value'] == 4
    assert read_excel('BookC.xlsx').i[0, 0] == 3
    
    with open_excel('BookA.xlsx') as a, open_excel('BookB.xlsx') as b, open_excel('BookC.xlsx') as c:
        c[0]['B2'] = 41
        a.save()
        b.save()
        c.save()
    assert read_excel('BookA.xlsx')['link to cell with link to other workbook', 'value from link'] == 42
    assert read_excel('BookB.xlsx')['cell with link to other workbook', 'formula value'] == 42
    assert read_excel('BookC.xlsx').i[0, 0] == 41    
    with open_excel('BookA.xlsx') as a, open_excel('BookB.xlsx') as b, open_excel('BookC.xlsx') as c:
        c[0]['B2'] = 3
        a.save()
        b.save()
        c.save()
    assert read_excel('BookA.xlsx')['link to cell with link to other workbook', 'value from link'] == 4
    assert read_excel('BookB.xlsx')['cell with link to other workbook', 'formula value'] == 4
    assert read_excel('BookC.xlsx').i[0, 0] == 3

BookA.xlsx
BookB.xlsx
BookC.xlsx