fix: misc fixes in pub/gpg/tests (#38809)

- The pub registry reported the oldest version as `latest`, because the
descriptor slice is sorted ascending but the first element was used.
- Verifying a GPG or SSH key flashed success and redirected after
already writing an error response, so a failure was reported as a
success with an empty key id.
- Test packages sharing redis could tear down each other's server.
`PrepareTestRedis` started its own on the well-known port, so a package
running in parallel borrowed it and lost it when the owner's cleanup
fired. It now listens on a socket of its own.

routers/api/packages/pub/pub.go

@@ -108,7 +108,7 @@ func EnumeratePackageVersions(ctx *context.Context) {
 
 	jsonResponse(ctx, http.StatusOK, &packageVersions{
 		Name:     pds[0].Package.Name,
-		Latest:   packageDescriptorToMetadata(baseURL, pds[0]),
+		Latest:   versions[len(versions)-1], // versions mirrors pds, sorted ascending
 		Versions: versions,
 	})
 }