test: database_test: fix threading confusion
test: database_test: fix threading confusion

database_test contains several instances of calling do_with_cql_test_env()
with a function that expects to be called in a thread. This mostly works
because there is an internal thread in do_with_cql_test_env(), but is not
guaranteed to.

Fix by switching to the more appropriate do_with_cql_test_env_thread().

Closes #7333

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