tests: Add a gst-tester utility
gst-tester is a tool to launch .validatetest files with
TAP[0] compatible output and supporting missing gst-validate
application which means that it can be cleanly integrated with meson
test harness.
It allows us to use gst-validate to write integration tests in any
GStreamer repository keeping them as close as possible to the code. It
can simplify a lot test writing and reading and not having to go into
another repository to implement or run tests makes it more convenient to
use.
This also implements a stupid simple test to show how that works