Make run_tests print a warning when skip_proofs is set in .isabelle/etc #905
Description
In rare circumstances in our Isabelle fork, the "Skip proofs" checkbox's value can be saved to ~/.isabelle/etc/settings. This leads to all commandline builds resulting in proofs being skipped, and it's very hard to spot this situation (interactive proofs failing, but commandline passing, or one machine's tests passing while another's fail).
We should check for this in run_tests and print a big red warning when running with skip proofs on.
isabelle options -g skip_proofs should produce true or false accordingly