integration: add upstream validator to OCI roundtrip tests

In order to make sure that we don't create invalid OCI images that are
consistently invalid, add additional checks to ensure that both of the
generated OCI images in the round-trip test are valid according to the
upstream validator.

This commit vendors the following packages (deep breath):
* oci/image-tools@7575a09363, which requires
* oci/image-spec@v1.0.0-rc4 [revendor, but is technically an update
  because I couldn't figure out what version was vendored last time]
* oci/runtime-spec@v1.0.0-rc4
* xeipuuv/gojsonschema@6b67b3fab7
* xeipuuv/gojsonreference@e02fc20de9
* xeipuuv/gojsonpointer@e0fe6f6830
* camlistore/go4@7ce08ca145

Signed-off-by: Aleksa Sarai <asarai@suse.de>

vendor/github.com/opencontainers/runtime-spec/specs-go/version.go

@@ -0,0 +1,18 @@
+package specs
+
+import "fmt"
+
+const (
+	// VersionMajor is for an API incompatible changes
+	VersionMajor = 1
+	// VersionMinor is for functionality in a backwards-compatible manner
+	VersionMinor = 0
+	// VersionPatch is for backwards-compatible bug fixes
+	VersionPatch = 0
+
+	// VersionDev indicates development branch. Releases will be empty string.
+	VersionDev = "-rc4"
+)
+
+// Version is the specification version that the package types support.
+var Version = fmt.Sprintf("%d.%d.%d%s", VersionMajor, VersionMinor, VersionPatch, VersionDev)