casts test: more info on failure

print input and output masks and the destination type on failure

Signed-off-by: Matthias Kretz <kretz@kde.org>
unittest: skip unnecessary IndexType usage

Signed-off-by: Matthias Kretz <kretz@kde.org>
simd_cast: all cast tests compile and pass

I implemented several casts between masks of different sizes and
implementations (Scalar, SSE, simd_mask_array). There was some partially
incorrect code here before where some simd_cast functions matched too
greedily.

Signed-off-by: Matthias Kretz <kretz@kde.org>