contrib: wire up osxkeychain in contrib/Makefile on macOS

contrib: wire up osxkeychain in contrib/Makefile on macOS

When running "make test" with TEST_CONTRIB_TOO=yes (which is default in
macOS CI workflows), $(MAKE) -C contrib/ test is invoked. However,
contrib/Makefile only invoked tests for diff-highlight and subtree,
meaning git-credential-osxkeychain was never built or verified during
standard CI test runs.

Add a "test" target to contrib/credential/osxkeychain/Makefile that
depends on building git-credential-osxkeychain. Additionally, wire up
credential/osxkeychain in contrib/Makefile under "all", "test", and
"clean" whenever running on macOS (Darwin).

This ensures that running "make test" or "make all" in contrib on macOS
automatically builds and links git-credential-osxkeychain, preventing
future build or symbol linking regressions from slipping through CI.

Signed-off-by: Shardul Natu <snatu@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>