Change 'require' to accept a list of predicates

Change 'require' to accept a list of predicates

This changes 'require' to accept a list of simple predicates.  For
now, each predicate is just the name of a proc, optionally prefixed
with "!" to indicate that the result should be inverted.

It's possible to make this fancier, but so far I haven't done so.  One
idea I had is to allow a predicate to have associated text to display
on failure.  Another is to convert the predicates that need a running
gdb (e.g., skip_python_tests) to start their own gdb, and then
'require' could enforce the rule that gdb not be running when it is
called.