git: disable failing test on darwin

The tests for null patterns where changed in 25754125cef278c7e9492fbd6dc4a28319b01f18,
it's possible utf-8 normalisation is causing different behaviour here.

    not ok 54 - LC_ALL='C' git grep -P -f f -i 'Æ<NUL>[Ð]' a
    not ok 57 - LC_ALL='C' git grep -P -f f -i '[Æ]<NUL>Ð' a
    not ok 60 - LC_ALL='C' git grep -P -f f -i '[Æ]<NUL>ð' a
    not ok 63 - LC_ALL='C' git grep -P -f f -i 'Æ<NUL>Ð' a
    Dubious, test returned 1 (wstat 256, 0x100)
    Failed 4/145 subtests
            (less 48 skipped subtests: 93 okay)