Makefile.monolith: change [make unattended] to use random testing mode and to stop after MAX scenarios have been found.

API change: use a [settings] record instead of a bunch of parameters. Expose just [run] instead of [run_afl] and [run_random].

Suggested by Gabriel Scherer.

[main_random]: use just one clock for all tests, as opposed to a new clock every time we switch to a smaller value of [fuel].

This was useless and did not use the correct switch.