git: disable failing test on darwin

[LnL7]

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)

Fixes #73806

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[wmertens]

I'm not ecstatic about disabling tests on Darwin without having an issue open to fix the tests properly.

After all, even though the tested behavior seems to be a rare cornercase, it's still something that no longer behaves the way it should.

[LnL7]

I agree with your concern, but these are new tests so either the test behaves a bit different or the null behaviour was only fixed on linux (no regression). Let's just re-evaluate with the next update similar to t1700-split-index.