sh: Add more testcases for ${var:-word}.
sh: Add more testcases for ${var:-word}.

Whether POSIX requires these is unclear.

They pass with 8-stable sh as well.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase