testsuite: use 'fatal_' and 'framework_failure_' for hard errors

testsuite: use 'fatal_' and 'framework_failure_' for hard errors

* tests/defs (require_xsi): Use `fatal_', not `framework_failure',
to report an invalid usage.
* tests/remake-gnulib-remove-header.test: Prefer using `fatal_'
with a proper error message over a direct call to `Exit 99'.
* tests/pr8365-remake-timing.test: Likewise.
* tests/cygnus-imply-foreign.test: Likewise.
* tests/missing6.test: Likewise.
* tests/cond8.test: Likewise.
* tests/cond33.test: Likewise.
* tests/python-virtualenv.test: Prefer using `framework_failure_'
with a proper error message over a direct call to `Exit 99'.
* tests/instspc-tests.sh: Prefer using `framework_failure_' and
`fatal_' over direct calls to `Exit 99'.
(fatal_): Define this (which is a simplified version of the one
in `tests/defs') for early uses (i.e., before `tests/defs'
gets sourced).
* tests/depmode-tests.sh: Likewise.  Also, simplify the
'get_depmodes' function and calls to it accordingly.