Conformance teardown test deleteManifestBeforeBlobs should allow return 400 when delete by tag is disallowed by registry

[MinerYang]

when running deleteManifestBeforeBlobs tear down test, contains delete by tag reference, which some of the registry is disallowed delete manifest by tag, according to https://github.com/opencontainers/distribution-spec/blob/v1.1.0/spec.md#deleting-tags, it should also include the Equal(http.StatusBadRequest) to satisfy the conditions.

distribution-spec/conformance/03_discovery_test.go

Line 409 in 0f98d91

Equal(http.StatusMethodNotAllowed),

[sudo-bmitch]

@MinerYang thanks for the report. I've opened up #525 which I believe resolves this. Let us know if that works for you.