Add testcases for -i with hardlinks and symlinks

The symlink testcase is expected to fail, post-r312519 (the revert of
r312404); mark it so.

MFC after:	3 weeks
Sponsored by:	Dell EMC Isilon

usr.bin/sed/tests/Makefile

@@ -2,6 +2,7 @@
 
 PACKAGE=	tests
 
+ATF_TESTS_SH+=	sed2_test
 NETBSD_ATF_TESTS_SH+=	sed_test
 TAP_TESTS_SH=	legacy_test
 TAP_TESTS_SH+=	multi_test

usr.bin/sed/tests/sed2_test.sh

@@ -0,0 +1,61 @@
+#
+# Copyright 2017 Dell EMC.
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+#   notice, this list of conditions and the following disclaimer.
+# * Redistributions in binary form must reproduce the above copyright
+#   notice, this list of conditions and the following disclaimer in the
+#   documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#
+# $FreeBSD$
+#
+
+atf_test_case inplace_hardlink_src
+inplace_hardlink_src_head()
+{
+	atf_set "descr" "Verify -i works with a symlinked source file"
+}
+inplace_hardlink_src_body()
+{
+	echo foo > a
+	atf_check ln a b
+	atf_check sed -i '' -e 's,foo,bar,g' b
+	atf_check -o 'inline:bar\n' -s exit:0 cat b
+}
+
+atf_test_case inplace_symlink_src
+inplace_symlink_src_head()
+{
+	atf_set "descr" "Verify -i works with a symlinked source file"
+}
+inplace_symlink_src_body()
+{
+	atf_expect_fail "Check for S_IFREG reverted in r312404"
+
+	echo foo > a
+	atf_check ln -s a b
+	atf_check -e not-empty -s not-exit:0 sed -i '' -e 's,foo,bar,g' b
+}
+
+atf_init_test_cases()
+{
+	atf_add_test_case inplace_hardlink_src
+	atf_add_test_case inplace_symlink_src
+}