Subtract two values based on Rank

Ok so in that case the visual must already have been filtered to the test type. If you have multiple tests per person, within the same type, how are you filtering it to show the 2 most recent results? If you are not filtering it, and you are using measures to display columns 2 & 3 of your visual, you'll need to factor this into the DAX, I suggest something like this:

VAR _name =
    SELECTEDVALUE ( 'Table'[Name] )
VAR _test_type = 
    SELECTEDVALUE( 'Table'[Test] )
VAR _recent_test = 
    MAX ( 'Table'[Testrang] )
VAR _recent_test_score = 
    CALCULATE (
        SUM ( 'Table'[Wert] ),
        REMOVEFILTERS ( 'Table'[Testrang] ),
        'Table'[Testrang] = _recent_test
            && 'Table'[Name] = _name
            && 'Table'[Test] = _test_type
    )
VAR _prev_test = _recent_test - 1
VAR _prev_test_score =
    CALCULATE (
        SUM ( 'Table'[Wert] ),
        REMOVEFILTERS ( 'Table'[Testrang] ),
        'Table'[Testrang] = _prev_test
            && 'Table'[Name] = _name
            && 'Table'[Test] = _test_type
    )
VAR _current_minus_prev =
    _recent_test_score - _prev_test_score
RETURN
    _current_minus_prev

If this works for you please accept as the solution for visibilty of others.