Add test for properties named $ref

[matt-allan]

The JSON Reference spec states:

A JSON Reference is a JSON object, which contains a member named
"$ref", which has a JSON string value. Example:

{ "$ref": "http://example.com/example.json#/foo/bar" }

If a JSON value does not have these characteristics, then it SHOULD
NOT be interpreted as a JSON Reference.

The swagger schema declares a property named $ref that is not actually a reference, like this:

"properties": {
        "$ref": {
          "type": "string"
        }
      }

Since the value is not a string, it's not considered a reference and should not be dereferenced.

This test ensures that a validator will not attempt to dereference a property named $ref when the value is not a string.

[Julian]

Might belong a bit more in the properties tests than the ref tests, can't really decide, but thanks!

[epoberezkin]

This test ensures that a validator will not attempt to dereference a property named $ref when the value is not a string.

"when the value is not a string" is an unnecessary qualifier here, the validators simply should not assign any special meaning to properties of properties keyword, everything is just a property name, and unless its value is an object (and a valid schema) the schema is invalid (e.g. if $ref is a string here).

[matt-allan]

To be clear, the intention of the test is to to ensure that the validator will not attempt to dereference any property named $ref when the value is not a string. I was just explaining what this specific test is doing. This test places the $ref under properties but it could be anywhere in the schema.