test: cql_test_env: enable features in a `seastar::thread`
test: cql_test_env: enable features in a `seastar::thread`

Each feature can have an associated `when_enabled` callback
registered, which is assumed to run in the thread context,
so wrap the `enable()` call in a seastar thread.

Tests: unit(dev)

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

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