simdarray: remove begin/end again (instead: Vc::begin/end)

the non-member begin/end in iterator.h are much more generic and should
implement what is needed for simdarray. Therefore better remove the
member begin/end (which are kind of wrong for MIC at least)

Signed-off-by: Matthias Kretz <kretz@kde.org>
unittest: make it easier to compile just a single test

if you define UNITTEST_ONLY_XTEST now, then only XTEST* functions will
be compiled (i.e. the reverse behavior than normal)

I don't know how to implement this without code duplication. :(

Signed-off-by: Matthias Kretz <kretz@kde.org>
simdarray: use const-ref parameters

use const-ref for "generic" simdarray parameters, as otherwise
overalignment leads to warnings.

Signed-off-by: Matthias Kretz <kretz@kde.org>
AVX::Mask: fix (u)short_m compare operators

it was using movemask_ps and thus ignoring every second value

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