perlPackages.IOTty: fix darwin sandbox build

2 of the tests fail when sandboxing is enabled, we might want to allow
this case in the profile if there are more cases but for now just
disable the tests on darwin.

    warning: TIOCSCTTY failed, slave might not be set as controlling terminal: Operation not permitted at /private/tmp/nix-build-perl5.28.1-IO-Tty-1.12.drv-0/IO-Tty-1.12/blib/lib/IO/Pty.pm line 121.
    Error: could not connect pty as controlling terminal!
    no controlling terminal at t/test.t line 68.
    sysread() failed:  at t/test.t line 91.
    # Looks like your test exited with 255 just after 2.