tests: update JSON cql tests
tests: update JSON cql tests

Tests are updated to check for recently fixed issues, i.e.
 * proper handling of null values
 * parsing decimal values from string

Refs #3664
Refs #3666
Refs #3667

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