Add additional testcases for cat(1)

Verify the following additional cases:

• -s (in isolation, in addition to the -se testcase obtained via the NetBSD test).
• -vt

Submitted by: shivansh
Reviewed by: asomers (earlier diff), ngie
MFC after: 1 month
Sponsored by: Google, Inc (GSoC 2017)
Differential Revision: D11020