tests/cql_query_test.cc: convert some tests to thread
tests/cql_query_test.cc: convert some tests to thread

Some tests are currently discarding futures unjustifiably, however
adding code to wait on these futures is quite inconvenient due to the
continuation style code of these tests. Convert them to run in a seastar
thread to make the fix easier.

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