Fixed the compilation with xbuild

```
CSC: error CS2001: Source file `Constraints/AllItemsConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/AssignableFromConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/AttributeExistsConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/BinarySerializableTest.cs' could not be found
CSC: error CS2001: Source file `Constraints/CollectionContainsConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/CollectionEquivalentConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/CollectionOrderedConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/DelayedConstraintTest.cs' could not be found
CSC: error CS2001: Source file `Constraints/AndConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/EndsWithConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/ExactCountConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/ExactTypeConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/InstanceOfTypeConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/NUnitComparerTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/ComparisonConstraintTest.cs' could not be found
CSC: error CS2001: Source file `Constraints/GreaterThanOrEqualConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/GreaterThanConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/LessThanOrEqualConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/LessThanConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/NotConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/OrConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/RangeConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/StartsWithConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/XmlSerializableTest.cs' could not be found
CSC: error CS2001: Source file `Constraints/SubstringConstraintTests.cs' could not be found
CSC: error CS2001: Source file `Constraints/AssignableToConstraintTests.cs' could not be found

 9 Warning(s)
 26 Error(s)
```