When using subtimeline serialized with the command line formatter
syntax, we had a false positive when detecting if the user had explicitly
specified tracks with the `+track` syntax. Verifying the presence of
`+track` explicitly in the `args` array ensure the `+track` is not for
a subtimeline.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5981>