The "switch-track" command can be used to switch tracks. The "type"
argument selects which track type to change (can be "audio", "video",
or "text"). The "index" argument selects which track of this type
to use: it can be either a number, which will be the Nth track of
the given type, or a number with a "+" or "-" prefix, which means
a relative change (eg, "+1" means "next track", "-1" means "previous
track").
Conflicts:
validate/gst/validate/gst-validate-scenario.c
