Changed tests and solution file for NUnit 2.5+.

Everything that was in the `NUnit.Framework.SyntaxHelpers` namespace is now simply a part of `NUnit.Framework`, as of NUnit 2.5 (see http://nunit.com/index.php?p=releaseNotes&r=2.5).

A minor change was required to the csproj were required to get the solution to build with a different version of NUnit.