test: stabilize named context transfer tracking

[tonistiigi]

Track transfer progress by vertex digest instead of parsing the status ID. This avoids conflating separate local context syncs that share a display name.

Should fix flaky:

    === FAIL: frontend/dockerfile TestIntegration/TestNamedFilteredContext/worker=oci-snapshotter-stargz/frontend=client/new/all (0.90s)
        dockerfile_test.go:9044: 
            	Error Trace:	/src/frontend/dockerfile/dockerfile_test.go:9044
            	Error:      	Received unexpected error:
            	            	not enough data was transferred, 33 < 4096000
            	            	github.com/moby/buildkit/frontend/dockerfile.testNamedFilteredContext.func1.1.2
            	            		/src/frontend/dockerfile/dockerfile_test.go:9036
            	            	golang.org/x/sync/errgroup.(*Group).Go.func1