Merge branch 'jc/test-i18ngrep'

With release 2.44 we got rid of all uses of test_i18ngrep and there
is no in-flight topic that adds a new use of it.  Make a call to
test_i18ngrep a hard failure, so that we can remove it at the end
of this release cycle.

* jc/test-i18ngrep:
  test_i18ngrep: hard deprecate and forbid its use

diff --cc t/test-lib-functions.sh
Simple merge