Make End() return the position after the node

End() was previously only used to determine if a comment was within
a Node, so it used the expedient definition of the position of the
last token in the node.  In the next patch it will be used for
capturing substrings of the Blueprint file, so make it point to
the character after the last token instead.

Also add tests for it.

Test: parser_test.go
Change-Id: Icaff3915b41e251ef9d0aad5615021bf37406aee