[SIMD] Test the index vectors the same way we test other vectors.

On my machine (paranoia, ivy bridge) with gcc 6.3, -O0 -ggdb, 4 cores, and Vc this reduces the compilation time for the full test suite. Not as much as I hoped, but at least a little.

The other reason for this change is to reduce the surprise because of missing expected functionality a little.

Before:

real	14m22,836s
user	44m53,222s
sys	1m41,429s

After:

real	11m55,741s
user	38m47,796s
sys	1m29,003s

Closes: #125 (closed).

WIP: make sure vectorclass can handle this before merging.