manifest_test.go: Improve the test content
manifest_test.go: Improve the test content

Signed-off-by: zhouhao <f577ade201aa03d612c19915a089dc6084d45cd9@cn.fujitsu.com>

vbatts/oci-image-spec,opencontainers/image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec