tests: No implicit return in "Type text" step"

Remove return at the end of "Type text" step.

Port all original "Type text" using steps that need return to "Type text
and return" step instead.

Also update README to reflect the fact that xdotool is now a general
dependency and not just needed by "Send key combos" test.

https://bugzilla.gnome.org/show_bug.cgi?id=748006