simd_for_each test: more thorough test of write-back
simd_for_each test: more thorough test of write-back

have the test check that write-back to the container is only done for
the lvalue-ref lambda

Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,chr-engwer/Vc