Is the point that sytest assumes each test was run in isolation, so that there were no other rooms to search for?

Exactly, Sytest is creating separate a user and room for each test, so would only receive the results from those rooms.
Since we're only using Alice and a new room for each test, we need to filter on the room.

It looks like the sytest has some extra checks here. Do we want to carry those over?

• next_batch from assert_json_keys( $room_events, qw( count results next_batch ) );
• context from

         my $context = $results->[0]{context};
         assert_json_keys( $context, qw( events_before events_after ));
         my $events_before = $context->{events_before};
         my $events_after = $context->{events_after};
  
         assert_eq( scalar @$events_before, 2, 'events_before' );
         assert_eq( scalar @$events_after, 2, 'events_after' );

Oh, seems like I've removed one next_batch check too much.
For context: We're checking

sce := "search_categories.room_events"
result0 := sce + ".results.0.result"
resBefore := sce + ".results.0.context.events_before"
resAfter := sce + ".results.0.context.events_after"
[...]
match.JSONKeyEqual(resBefore+".0.content.body", "Message number 3"),
match.JSONKeyEqual(resBefore+".1.content.body", "Message number 2"),
match.JSONKeyEqual(resAfter+".0.content.body", "Message number 5"),
match.JSONKeyEqual(resAfter+".1.content.body", "Message number 6"),

which ensures context as well as the other keys (events_before/after) are present.

ahh sorry, I missed events_before and events_after. It does all start to blur together if you're not careful!