test framework: further deprecate test_i18ngrep

As an attempt to come up with a useful mechanism to ensure that
certain messages are left untranslated [*], we earlier wrote
GIT_TEST_GETTEXT_POISON off as a failed experiment.

But the output from the test helper was easier to use while
debugging failed tests, compared to the same test writtein with the
plain-vanilla "grep". Especially when a test that expects a certain
string to appear in the output (e.g. "this test must fail with this
message") fails, "grep message output" would just silently fail and
in a &&-chained sequence of commands, it is hard to tell which step
failed. test_i18ngrep explicitly said "we wanted to see a line that
match this pattern but did not see a hit in this file".

What we have as test_i18ngrep in our tree still retains this verbose
output (even though we got rid of the "poison" support). Let's
rename it to test_grep (because it is no longer about i18n at all)
and then make test_i18ngrep a thin wrapper around it. Existing
callers of test_i18ngrep can be mechanically rewritten to instead
use test_grep over time, but it does not have to be done in this
commit.

[Footnote]

* The idea was that human-facing messages are often translated, but
there are messages that should never be translated. We use
"grep" only for the latter kind of messages, and then run tests
in "poison" mode that spew garbage for translatable messages. If
such a test run fails, it means these messages tested with "grep"
were marked for translation by mistake. test_i18ngrep was to be
used for other messages that are to be translated, and was to
always "succeed" when runing under the "poison" mode.

Signed-off-by: Junio C Hamano <gitster@pobox.com>