For defining passthrough suites, we have util override files called set_read_preference_secondary.js, enable_causal_consistency.js, and enable_causal_consistency_without_read_pref.js. The second one includes the first one so that the default "causal consistency" override also sets secondary read preference.

enable_causal_consistency.js should be renamed to enable_causal_consistency_with_secondary_read_pref.js to explicitly reflect the fact that it does set the read preference to secondary. (I just spent a while going down a rabbit hole trying to understand how the aggregation_secondary_reads suite enables secondary reads without loading set_read_preference_secondary.js directly.)