The old API would only assert or return an invalid timecode, the new API
returns a boolean or NULL. We can't change the existing API
unfortunately but can at least deprecate it.
In gst_video_time_code_is_valid, also check for invalid
ranges when using drop-frame TC. Refactor some code which
broke after the check was added.
https://bugzilla.gnome.org/show_bug.cgi?id=779010
Comparing floats for equality is not necessarily going to
work reliably, so use fail_unless_equals_float() for this.
Test would fail on x86 (Intel Atom x5-Z8300).
