Provide harness to run illumos's oclo tests from libc regress

This depends on the illumos-os-tests port I just imported and can be
linked to the build once guenther lands the close-on-fork diff.

Adapted from an initial diff by Ricardo Branco

Author: botovq

regress/lib/libc/illumos/Makefile

@@ -0,0 +1,7 @@
+#	$OpenBSD: Makefile,v 1.1.1.1 2025/08/02 06:16:34 tb Exp $
+
+SUBDIR += oclo
+
+install:
+
+.include <bsd.subdir.mk>

regress/lib/libc/illumos/Makefile.inc

@@ -0,0 +1,9 @@
+#	$OpenBSD: Makefile.inc,v 1.1.1.1 2025/08/02 06:16:34 tb Exp $
+
+ILLUMOS_OS_TESTDIR = /usr/local/share/illumos-os-tests
+
+.if !exists(${ILLUMOS_OS_TESTDIR})
+regress:
+	@echo package illumos-os-tests is required for this regress
+	@echo SKIPPED
+.endif

regress/lib/libc/illumos/oclo/Makefile

@@ -0,0 +1,16 @@
+#	$OpenBSD: Makefile,v 1.1.1.1 2025/08/02 06:16:34 tb Exp $
+
+.if exists(/usr/local/share/illumos-os-tests)
+
+PROGS =		oclo
+PROGS +=	oclo_errors
+PROGS +=	ocloexec_verify
+
+LDADD_ocloexec_verify = -lkvm
+
+WARNINGS =	yes
+
+.PATH: /usr/local/share/illumos-os-tests/tests/oclo
+.endif
+
+.include <bsd.regress.mk>