check: fix jitterbuffer check
check: fix jitterbuffer check

Don't advance the clock to 240ms too early.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=710013

jhodapp/gst-plugins-good,jhodapp/gst-plugins-good,jhodapp/gst-plugins-good,jhodapp/gst-plugins-good