alternator-tests: two simple test for nested documents
alternator-tests: two simple test for nested documents

This patch adds two simple tests for nested documents, which pass:

test_nested_document_attribute_overwrite() tests what happens when
we UpdateItem a top-level attribute to a dictionary. We already tested
this works on an empty item in a previous test, but now we check what
happens when the attribute already existed, and already was a dictionary,
and now we update it to a new dictionary. In the test attribute a was
{b:3, c:4} and now we update it to {c:5}. The test verifies that the new
dictionary completely replaces the old one - the two are not merged.
The new value of the attribute is just {c:5}, *not* {b:3, c:5}.

The second test verifies that the AttributeUpdates parameter of
UpdateItem cannot be used to update a just a nested attributes.
Any dots in the attribute name are considered an actual dot - not
part of a path of attribute names.

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

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