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)