debugutils: clockselect, a pipeline that enables clock selection

Sometimes, one wants to force a clock on some pipelines - for instance, when testing TSN related pipelines, one usually uses GstPtpClock or CLOCK_REALTIME (assuming system realtime clock is in sync with network one). Until now, one needs to write an application for that - not difficult, but quite boring if one just wants to test something. This patch presents a new element to help that: clockselect.

clockselect is a pipeline with two properties to select a clock. One property, "clock-id", enables one to choose between "monotonic", "realtime", "ptp" or "default" clock - where default keeps pipeline behaviour of choosing a clock based on its elements. The other property, "ptp-domain" gives one the choice of which PTP domain should be used.

Some very simple tests also added for this new element.