[D-runtime] druntime tests do not pass without -release

[David Nadlinger]

While tracing down an issue, I tried building druntime tests without 
release. They fail, on all platforms I tested (Windows, Linux, OS X).

The reason they don't during normal test runs is that the assertion 
failures are in contracts and invariants, which are disabled when using 
-release.

I suggest immediately removing the flag for unittest builds (or running 
them both with release and without), so that we can then work on 
determining whether the uncovered failures are real bugs or not with 
auto-tester support.

David