Watch TestRegistryAsCacheMutationAPIs for Flakes

I'm seeing this test fail occasionally with a timeout after 30 seconds:

=== RUN   TestRegistryAsCacheMutationAPIs
Jul 28 22:12:22 |WARN| No HTTP secret provided - generated random secret. This may cause problems with uploads if multiple registries are behind a load-balancer. To provide a shared secret, fill in http.secret in the configuration file or set the REGISTRY_HTTP_SECRET environment variable. go_version=go1.13.14
Jul 28 22:12:22 |INFO| redis not configured
Jul 28 22:12:22 |INFO| Starting cached object TTL expiration scheduler...
--- FAIL: TestRegistryAsCacheMutationAPIs (30.01s)
panic: Get http://example.com/v2/: dial tcp: i/o timeout [recovered]
        panic: Get http://example.com/v2/: dial tcp: i/o timeout

goroutine 9150 [running]:
testing.tRunner.func1(0xc0052c7c00)
        /usr/local/go/src/testing/testing.go:874 +0x3a3
panic(0xa60000, 0xc000bb4120)
        /usr/local/go/src/runtime/panic.go:679 +0x1b2
github.com/docker/distribution/registry/handlers.NewApp(0xc6d200, 0xc00002a0b8, 0xc0004b4800, 0xc002794ea0)
        /builds/project-0/registry/handlers/app.go:352 +0x2f3e
github.com/docker/distribution/registry/handlers_test.newTestEnvWithConfig(0xc0052c7c00, 0xc0004b4800, 0x1)
        /builds/project-0/registry/handlers/api_integration_test.go:4353 +0x5d
github.com/docker/distribution/registry/handlers_test.newTestEnvMirror(0xc0052c7c00, 0xc00279bf08, 0x1, 0x1, 0x0)
        /builds/project-0/registry/handlers/api_integration_test.go:4341 +0xeb
github.com/docker/distribution/registry/handlers_test.TestRegistryAsCacheMutationAPIs(0xc0052c7c00)
        /builds/project-0/registry/handlers/api_integration_test.go:4886 +0x7d
testing.tRunner(0xc0052c7c00, 0xb9a700)
        /usr/local/go/src/testing/testing.go:909 +0xc9
created by testing.(*T).Run
        /usr/local/go/src/testing/testing.go:960 +0x350
FAIL    github.com/docker/distribution/registry/handlers        228.180s

Example failed job: https://gitlab.com/gitlab-org/container-registry/-/jobs/660379092

This may have been introduced with: !289 (merged)