Make this test case accepts basename() in D script returns "" or "."

In Solaris, basename(1) and basename(3) both return "." while being given an
empty string (""), while in BSD (and Linux) basename(1) returns "" and
basename(3) returns "."

While here, also change #!/usr/bin/ksh to #!/usr/bin/env ksh to find ksh in
$PATH

Reviewed by: gnn, markj (earlier version), ngie (earlier version)
Differential Revision: https://reviews.freebsd.org/D11707