node tests: Avoid test leaks in filter.
node tests: Avoid test leaks in filter.

We want to be careful about MIT code, as it leads
to more lax checks in the filter code.

And then we just use with_field in a couple places
and clear subscriptions.

hackerkid/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,rht/zulip,rht/zulip,kou/zulip,punchagan/zulip,kou/zulip,rht/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,kou/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,kou/zulip