Implement workaround for regression - Fix Issue 11053.

http://d.puremagic.com/issues/show_bug.cgi?id=11053

This is a workaround for a regression, it seems using a `version(unittest)` block triggers some recursive instantiation errors in Phobos. But putting these lines in a simple `unittest` block fixes it.

See my comments here: https://github.com/D-Programming-Language/phobos/pull/1561/files#r6389254