manifest: improve test output and use const
						
							
							Use consts to make clear these values are fixed, and improve the output
to make it clearer which part is the expected output, and which part
the actual.
Before this:
    === RUN   TestManifest
        manifest_test.go:87: manifest bytes not equal: "{\n   \"schemaVersion\": 2,\n   \"mediaType\": \"application/vnd.oci.image.manifest.v1+json\",\n   \"config\": {\n      \"mediaType\": \"application/vnd.oci.image.config.v1+json\",\n      \"size\": 985,\n      \"digest\": \"sha256:1a9ec845ee94c202b2d5da74a24f0ed2058318bfa9879fa541efaecba272e86b\",\n      \"annotations\": {\n         \"apple\": \"orange\"\n      }\n   },\n   \"layers\": [\n      {\n         \"mediaType\": \"application/vnd.oci.image.layer.v1.tar+gzip\",\n         \"size\": 153263,\n         \"digest\": \"sha256:62d8908bee94c202b2d35224a221aaa2058318bfa9879fa541efaecba272331b\",\n         \"annotations\": {\n            \"lettuce\": \"wrap\"\n         }\n      }\n   ],\n   \"annotations\": {\n      \"hot\": \"potato\"\n   }\n}" != "{\n   \"schemaVersion\": 2,\n   \"mediaType\": \"application/vnd.oci.image.manifest.v1+json\",\n   \"config\": {\n      \"mediaType\": \"application/vnd.oci.image.config.v1+json\",\n      \"size\": 985,\n      \"digest\": \"sha256:1a9ec845ee94c202b2d5da74a24f0ed2058318bfa9879fa541efaecba272e86b\",\n      \"annotations\": {\n         \"apple\": \"orange\"\n      }\n   },\n   \"layers\": [\n      {\n         \"mediaType\": \"application/vnd.oci.image.layer.v1.tar+gzip\",\n         \"size\": 153263,\n         \"digest\": \"sha256:62d8908bee94c202b2d35224a221aaa2058318bfa9879fa541efaecba272331b\",\n         \"annotations\": {\n            \"lettuce\": \"wrap\"\n         }\n      }\n   ],\n   \"annotations\": {\n      \"hot\": \"potato\"\n   }\n}"
    --- FAIL: TestManifest (0.00s)
After this:
    === RUN   TestManifest
        manifest_test.go:72: manifest bytes not equal:
            expected:
            {
               "schemaVersion": 2,
               "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
               "config": {
                  "mediaType": "application/vnd.docker.container.image.v1+json",
                  "size": 985,
                  "digest": "sha256:1a9ec845ee94c202b2d5da74a24f0ed2058318bfa9879fa541efaecba272e86b"
               },
               "layers": [
                  {
                     "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
                     "size": 153263,
                     "digest": "sha256:62d8908bee94c202b2d35224a221aaa2058318bfa9879fa541efaecba272331b"
                  }
               ]
            }
            actual:
            {
               "schemaVersion": 2,
               "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
               "config": {
                  "mediaType": "application/vnd.docker.container.image.v1+json",
                  "size": 985,
                  "digest": "sha256:1a9ec845ee94c202b2d5da74a24f0ed2058318bfa9879fa541efaecba272e86b"
               },
               "layers": [
                  {
                     "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
                     "size": 153263,
                     "digest": "sha256:62d8908bee94c202b2d35224a221aaa2058318bfa9879fa541efaecba272331b"
                  }
               ]
            }
    --- FAIL: TestManifest (0.00s)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>