GCC 9.0: Fix ztest "directive argument is not a nul-terminated string"

GCC 9.0 is complaining because we're trying to print strings that
are defined like this:

.zo_pool = { 'z', 't', 'e', 's', 't', '\0' },

Fix them by making them actual strings.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #8330