node: Fix lightbox tests to use message_store.get().
node: Fix lightbox tests to use message_store.get().

While there are still a number of problems with these tests, they now
more effectively exercise the real code.

brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,brainwane/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,showell/zulip,hackerkid/zulip,showell/zulip,hackerkid/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,kou/zulip,zulip/zulip,zulip/zulip,showell/zulip,punchagan/zulip,brainwane/zulip,hackerkid/zulip,shubhamdhama/zulip,punchagan/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,hackerkid/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,andersk/zulip,punchagan/zulip,shubhamdhama/zulip,brainwane/zulip,shubhamdhama/zulip,punchagan/zulip,synicalsyntax/zulip,shubhamdhama/zulip,rht/zulip,kou/zulip,punchagan/zulip,kou/zulip,kou/zulip,synicalsyntax/zulip,andersk/zulip,synicalsyntax/zulip,showell/zulip,timabbott/zulip,rht/zulip,timabbott/zulip,showell/zulip,timabbott/zulip,brainwane/zulip,kou/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,shubhamdhama/zulip,timabbott/zulip,andersk/zulip,andersk/zulip,brainwane/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,shubhamdhama/zulip,synicalsyntax/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,timabbott/zulip,timabbott/zulip,synicalsyntax/zulip