Add a testcase for ln -sF

The testcase fails today, so mark it with atf_expect_fail: in
particular, the target (B) isn't being unlinked and the documentation
doesn't suggest special handling for directories. Thus, there's either
a doc or an implementation bug in ln(1) that needs to be resolved.

MFC after:	1 month
MFC with:	r319714, r319854, r319855
PR:		219943
Reviewed by:	ngie
Submitted by:	shivansh
Differential Revision:	D11159 (part of a larger diff)
Sponsored by:	Google, Inc (GSoC 2017)

bin/ln/tests/ln_test.sh

@@ -150,6 +150,22 @@ snf_flag_dir_body()
         atf_check -o inline:'B\n' readlink C
 }
 
+atf_test_case sF_flag
+sF_flag_head()
+{
+        atf_set "descr" "Verify that if the target file already exists " \
+                        "and is a directory, then '-sF' option removes " \
+                        "it so that the link may occur"
+}
+
+sF_flag_body()
+{
+	atf_expect_fail "B isn't being unlinked (bug 219943)"
+	atf_check mkdir A B
+        atf_check ln -sF A B
+        atf_check -o inline:'Symbolic Link\n' stat -f %SHT B
+}
+
 atf_test_case sf_flag
 sf_flag_head()
 {
@@ -220,6 +236,7 @@ atf_init_test_cases()
         atf_add_test_case target_exists_symbolic
         atf_add_test_case shf_flag_dir
         atf_add_test_case snf_flag_dir
+        atf_add_test_case sF_flag
         atf_add_test_case sf_flag
         atf_add_test_case s_flag
         atf_add_test_case s_flag_broken