t5523: use test_i18ngrep for negation

t5523: use test_i18ngrep for negation

Replace the first form with the second one:

	! grep expected actual
	test_i18ngrep ! expected actual

The latter syntax is supported by test_i18ngrep defined in
t/test-lib.sh.

Although the test already passes whether GETTEXT_POSION is enabled, use
the i18n grep variant for the sake of consistency and also to make
obvious that those strings are subject to i18n.

Signed-off-by: Vasco Almeida <redacted>
Signed-off-by: Junio C Hamano <redacted>