test defs: new function 'fatal_', for hard errors

test defs: new function 'fatal_', for hard errors

Before this patch, the only way offered by tests/defs 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.

Inspired by a recent similar change to Gnulib's tests/init.sh.

* tests/defs.in (fatal_): New function.
* tests/README (Section "Writing test cases" subsection "Do"):
Suggest the use of `fatal_', not of `framework_failure_', for
generic hard errors.  The latter should be reserved for "real"
set-up failures.