[mono.data.sqlite] Fixed the unit tests for building using make
 - Added the AssertHelpers to the test .sources
 - Assert.IsInstanceOf does not exist in the NUnit used, so switched it out for an older way
 - Marked DataColumnTest.ChangeTypeAfterSettingDefaultValue as NotWorking as the original test was invalid and after correction it fails (see bug #20418)
 - fixed some line endings in generated xml
 - Corrected the unit test for reading xml columns (using IXmlSerializable)