Issue 7848 - pure and nothrow ignored on unittest blocks

http://d.puremagic.com/issues/show_bug.cgi?id=7848

Depends on #2141 (Essentially bug 7848 and [bug 8651](http://d.puremagic.com/issues/show_bug.cgi?id=8651) are unrelated, but it is necessary to avoid druntime unittest breaking).