Fix shebangs and execute bit of test scripts.

Since we don't have /usr/bin/ksh, use a generic way of specifying
ksh. Some of the tests only run with ksh93, so use this shell
for these tests. Two of the tests don't have the execute bit set,
so fix this, too.

Reviewed by: markj@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16270