test-lib: add missing prerequisites for Darwin

test-lib: add missing prerequisites for Darwin

commit d3d8c601fd ("t7815: fix unexpectedly passing test on macOS",
2025-06-02) added a MACOS prerequisite by adding a 'Darwin' case
label to the 'OS-specific' case statement. However, this commit
forgot to set several prerequisites which appear in the 'default'
case label, in addition to the new MACOS prerequisite. This causes
several tests, which macOS should pass, being skipped.

In order to run all applicable tests on macOS, add the missing
prerequisites to the 'Darwin' case.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>