validator/additionalItems: fix instanceLocation

[vnykmshr]

Fixes #252.

The additionalItems validation loop iterates over arr[evaluated:] which re-indexes from 0, but uses the loop index i directly in strconv.Itoa(i) for the instance location. This reports /0 instead of /2 (or whatever the correct offset is) in validation errors.

What changed

• validator.go: changed strconv.Itoa(i) to strconv.Itoa(evaluated+i) on line 378
• additionalitems_test.go: added TestAdditionalItemsErrorLocation using draft-04 schema to verify the error points to the correct array index

[santhosh-tekuri]

can you add test case on file based error messages. see invalid_schemas_test.go

this makes it easier to add similar tests easier to just adding to json file without any code change.

[vnykmshr]

Moved to a json-driven approach following invalid_schemas_test.go. Created a separate testdata/validation_errors.json + runner since this tests validation errors rather than compile errors. Is
that what you had in mind, or would you prefer the test case added to invalid_schemas.json directly?

[santhosh-tekuri]

I added error check support to existing test suite in 8d3372a

• change commit msg: validator/additionalItems: fix instanceLocation
• do git pull --rebase from boon branch
• remove validation_errors.json and validation_errors_test.go
• add file 'testdata/Extra-Test-Suite/tests/draft2019-09/additionalItems.json` with below content:

[
    {
        "description": "check index in error",
        "schema": {
            "$schema": "https://json-schema.org/draft/2019-09/schema",
            "items": [
                { "type": "string" },
                { "type": "string" }
            ],
            "additionalItems": {
                "type": "boolean"
            }
        },
        "tests": [
            {
                "description": "with fraction",
                "data": [ "a", "b", "c" ],
                "valid": false,
				"errors": [
					"at '/2' [S#/additionalItems/type]: got string, want boolean"
				]
            }
        ]
    }
]

[vnykmshr]

@santhosh-tekuri updated PR as per your review feedback