test/alternator: add test for key length limits
test/alternator: add test for key length limits

DynamoDB limits partition-key length to 2048 bytes and sort-key length
to 1024 bytes. Alternator currently has no such limits officially, but
if a user tries a key length of over 64 KB, the result will be an
"internal server error" as Alternator runs into Scylla's low-level key
length limit of 64 KB.

In this patch we add (mostly xfailing) tests confirming all the above
observations. The tests include extensive comments on what they are
testing and why. Some of these tests (specifically, the ones checking
what happens above 64 KB) should pass once Alternator is fixed. Other
tests - requiring that the limits be exactly what they are in DynamoDB -
may either not pass or change in the future, depending on what we decide
the limits should be in Alternator.

Refs #10347

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

Closes #10438

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla