Prototype a sysprof helper

Prototype a sysprof helper

This is an attempt to see how we can use sysprof data
in our tests to extract useful performance numbers.

Use it as a wrapper around any GTK+ process:

./testperf ./gtk4-widget-factory

Currently, it repeatedly runs the given commandline,
extracts the first css validation time from the resulting
syscap file, and prints out the min/max/avg of the runs
at the end.

This relies on the environment variable GTK_DEBUG_AUTO_QUIT
to cause the process to exit soon after launch.