The purpose of the tool has shifted from running a limited set
of performance tests repeatedly to collect performance data
and catch regressions, to a wrapper that drives gnome-shell via
scripts for tests.
With that, the concept of a "default" script doesn't really make
sense anymore.
Instead, turn the argument from an optional flag into a required
parameter. This will allow us to stop bundling the existing tests
in a follow-up commit.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2812>
e4da6a347b