markdown: Assert twitter previews render inside spoilers.
markdown: Assert twitter previews render inside spoilers.

Previously, we rendered the twitter previews outside of a
spoiler block at the end of the message. The commit series
ending with this commit fixes that by inlining twitter
previews instead of appending them all at the end. As a
consequence of the inlining, we have fixed the issue here.
This commit just adds a test to assert that.

Fixes #15518.

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