tests init: new function 'fatal_', for hard errors

tests init: new function 'fatal_', for hard errors

Before this patch, the only way offered by tests/init.sh to
properly signal a hard error was the `framework_failure_'
function.  But the error message issued by that function,
as its name would suggest, refers to a set-up failure in the
testsuite, while hard errors can obviously also be due to
other reasons.  The best way to fix this inconsistency is to
introduce a new function with a more general error message.

* tests/init.sh (fatal_): New function.