minor: Clarify that message_store is a deep test.
minor: Clarify that message_store is a deep test.

I believe the original message_store example used
to stub some things here, but the part of the tutorial
that pertains to stubbing is further down.

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