test(selector): add tests for "Supported Forms"

[smoyer64]

Supported forms testing

This PR provides a set of testcases to verify that the Selector behaves properly according to the "Supported Forms" table proposed in a GitHub issue at ucan-wg/delegation#5 (comment).

The test cases are divided into three groups:

• Tests that should return one or more IPLD datamodel.Nodes
• Tests that should return "null" which is currently replaced by nil in Go
• Tests that should return an error

Currently all the nil and error testcases pass. There are ten node testcases that are failing with two having obvious causes:

1. The "Negative Index" test properly Parses the Selector text but fails with an index out of range [-1] panic when Select is called.

2. The "Optional Iterator" test returns the expected values but also a nil value for the second element which isn't a List. This should probably be pruned during Select.

[hannahhoward]

I'm just going to merge this into @alanshaw 's branch. It looks helpful! Thank you so much @smoyer64

[fabiobozzo]

FYI: I prepared a PR with fixes for all broken "Pass" selector tests, currently listed in go-ucan: ucan-wg/go-ucan#18