Disable automatic dependencies in zfs-test package

All of the ZTS test scripts specify /bin/ksh as the interpreter.
Unfortunately, as of Fedora 27 only /usr/bin/ksh is provided by
the package manager. Rather than change all the scripts to
accommodate the latest Fedora disable automatic dependencies
for the zfs-test package. Functionally this will not cause
any problems since /bin is a symlink to /usr/bin.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6868