sed tests: Add a regression test for the c function

Based on the test case from PR 271817 by Mohamed Akram.

PR:		271817
MFC after:	2 weeks

usr.bin/sed/tests/sed2_test.sh

@@ -159,6 +159,28 @@ minus_e_body()
 	atf_check -o 'inline:--\nab\n' sed    $'1 i\\\n--' a
 }
 
+atf_test_case command_c
+command_c_head()
+{
+	atf_set "descr" "Verify that the 'c' command starts a new cycle"
+}
+command_c_body()
+{
+	printf "%s\n" a b c d e f > a
+	printf "%s\n" x c d e f > expected
+
+	atf_check -o file:expected sed '
+/a/,/b/c\
+x
+' a
+
+	atf_check -o file:expected sed '
+/a/,/b/c\
+x
+$!N
+' a
+}
+
 atf_test_case command_D
 command_D_head()
 {
@@ -182,5 +204,6 @@ atf_init_test_cases()
 	atf_add_test_case hex_subst
 	atf_add_test_case bracket_y
 	atf_add_test_case minus_e
+	atf_add_test_case command_c
 	atf_add_test_case command_D
 }