This run_test helper sets up a convention that allows
us to give really short tracebacks for errors, and
eventually we can have more control over running
individual tests. (The latter goal has some
complications, since we often intentionally leak
setup in tests.)
This appends a space when text is inserted at the end of a message
using `compose_ui.insert_syntax_and_focus`. This is definitely what
users expect when using this feature.
Fixes: #8569.
This fixes compose.test_video_link_compose_clicked to just
use a stub for compose_ui.insert_syntax_and_focus.
It also adds direct tests for compose_ui.insert_syntax_and_focus.
Fixes#6362
