Add tests for ln(1)

• Verify that when creating a hard link to a symbolic link, '-L' option creates a hard link to the target of the symbolic link
• Verify that when creating a hard link to a symbolic link, '-P' option creates a hard link to the symbolic link itself
• Verify that if the target file already exists, '-f' option unlinks it so that link may occur
• Verify that if the target file or directory is a symbolic link, '-shf' option prevents following the link
• Verify that if the target file or directory is a symbolic link, '-snf' option prevents following the link
• Verify that '-s' option creates a symbolic link
• Verify that '-w' option produces a warning if the source of a symbolic link does not currently exist

Submitted by: shivansh
Reviewed by: asomers, ngie
MFC after: 1 month
Sponsored by: Google, Inc (GSoC 2017)
Differential Revision: https://reviews.freebsd.org/D11084