alternator-test: comment out an error-path test that doesn't work on newer boto3
alternator-test: comment out an error-path test that doesn't work on newer boto3

Unfortunately, the boto3 library doen't allow us to check some of the
input error cases because it unnecessarily tests its input instead of
just passing it to Alternator and allowing Alternator to report the error.
In this patch we comment out a test case which used to work fine - i.e.,
the error was reported by Alternator - until recent changes to boto3
made it catch the problem without passing it to Alternator :-(

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20200330190521.19526-2-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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