check: Add API to filter g_warning/g_critical etc

check: Add API to filter g_warning/g_critical etc

New API functions to filter log messages before they are processed by
GstCheck. This can be used to discard specific messages that are
accepted by the test or to add callbacks that test specific messages.

Default bevavior when no callback is given to a filter is to discard the
message, because it does not makes sense to have a filter with no
callback which does not discard; that would be a noop.

Discarded messages will in addition to bypass the GstCheck handling also
return to GLib that the message is not fatal if it occurs.

https://bugzilla.gnome.org/show_bug.cgi?id=773091