test(cassandra): cap test container memory usage

Default memory usage is excessive.

.ci/docker-compose-file/cassandra/cassandra.yaml

@@ -469,8 +469,8 @@ concurrent_materialized_view_writes: 32
 # accepting writes when the limit is exceeded until a flush completes,
 # and will trigger a flush based on memtable_cleanup_threshold
 # If omitted, Cassandra will set both to 1/4 the size of the heap.
-# memtable_heap_space_in_mb: 2048
-# memtable_offheap_space_in_mb: 2048
+memtable_heap_space_in_mb: 2048
+memtable_offheap_space_in_mb: 2048
 
 # memtable_cleanup_threshold is deprecated. The default calculation
 # is the only reasonable choice. See the comments on  memtable_flush_writers

.ci/docker-compose-file/docker-compose-cassandra.yaml

@@ -12,6 +12,8 @@ services:
     environment:
       CASSANDRA_BROADCAST_ADDRESS: "1.2.3.4"
       CASSANDRA_RPC_ADDRESS: "0.0.0.0"
+      HEAP_NEWSIZE: "128M"
+      MAX_HEAP_SIZE: "2048M"
     volumes:
       - ./certs:/certs
     #ports: