std.variant: Adjust test for ARM

The array used in the test is bigger than the maximal size which can be stored in a Variant on ARM.
See http://d.puremagic.com/issues/show_bug.cgi?id=10879 for details.

Note: This is part of a set of changes which are required to get test suite & unit tests passing on ARM GDC. All necessary changes for GDC are here for reference:
https://github.com/jpf91/GDC/commits/arm-old

Once these pull requests are merged upstream I'll merge them into GDC as well.