tests: Add a gst-tester utility

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

[0] https://testanything.org/

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/461>