tests: cover the metadata file for multi exporter bake

[SaiPisey2]

testBakeMultiExporters checks the registry, docker store and OCI outputs, but leaves the metadata file untested:

// TODO: test metadata file when supported by multi exporters https://github.com/docker/buildx/issues/2181

Multiple exporters are supported now, and a bake with image, docker and OCI outputs does write a metadata file, so this asserts buildx.build.ref and containerimage.digest from it.

image.name is deliberately not asserted. With several named exporters it still reports only one name, so it cannot stand in for the others. The TODO now records that narrower gap instead of pointing at multi exporter support in general.

Testing

The integration harness does not come up on my machine, on master as well as on this branch: it fails in lazyMirrorRunnerFunc before any test body runs, so I could not exercise testBakeMultiExporters itself.

What I did check was the behaviour the assertions depend on, by running a bake with the same shape of outputs against buildx 0.32.1:

$ docker buildx bake --metadata-file md.json \
    --set "*.output=type=docker,name=buildx-mdtest:store" \
    --set "*.output=type=image,name=example.com/buildx/mdtest:reg" \
    --set "*.output=type=oci,dest=./result"

$ cat md.json
{
  "default": {
    "buildx.build.ref": "...",
    "containerimage.digest": "sha256:d4802c4a6163e307eaf90dddc0d9d977cce8f1910b4bb675681e4d8a9466f4cf",
    "containerimage.descriptor": { ... },
    "image.name": "example.com/buildx/mdtest:reg"
  }
}

Both asserted keys are present, and image.name carries only the type=image name while the type=docker one is missing, which is why it is left out of the assertions.