validate/validate_test: Handle JSON Schema test not raising an error
validate/validate_test: Handle JSON Schema test not raising an error

Without this, tests that expect an error tried to cast the nil to a
multierror (which failed) and then tried to format a failure message
with errs.Error() (which panicked).

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/runtime-tools,liangchenye/ocitools,wking/ocitools-v2,opencontainers/runtime-tools,opencontainers/ocitools,wking/ocitools-v2,opencontainers/ocitools,liangchenye/ocitools