alternator-test: test that index can't be a name reference (#xyz)
alternator-test: test that index can't be a name reference (#xyz)

We already have a test which shows verify DynamoDB and Alternator
do not allow an index in an attribute path - like a[0].b - to be
a value reference - a[:xyz].b. We forgot to verify that the index
also can't be a name reference - a[#xyz].b is a syntax error. So here
we add a test which confirms that this is indeed the case - DynamoDB
doesn't allow it, and neither does Alternator.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20210219123310.1240271-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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