Added workaround in test setup method that fail under some test runners

- Test.FullName nullrefs under some test runners
Added a few unit tests and fixed a few argument checks.
Added unit tests for SliceComparer
Added unit tests for SliceWriter + fixed a few corner cases

- SliceWriter.WriteVarBytes(Slice.Nil) will output 0x00
- SliceWriter indexer will throw if index is out of range