flaky test: TestDiffOpaqueDirsMerge/worker=oci-snapshotter-stargz

Don't think we have already seen this one failed yet: https://github.com/moby/buildkit/actions/runs/4076712153/jobs/7024876130#step:6:1544

logs.txt

=== CONT  TestIntegration/TestDiffOpaqueDirsMerge/worker=oci-snapshotter-stargz
    mergediff_test.go:1252: 
        	Error Trace:	/src/client/client_test.go:6227
        	            				/src/client/mergediff_test.go:1252
        	            				/src/client/run.go:197
        	Error:      	Received unexpected error:
        	            	process "sh -c chmod 0700 /modifyDir && rm -rf /unmodifiedDir/opaqueDir1 && mkdir -p /unmodifiedDir/opaqueDir1/newOpaqueSubdir1 && touch /unmodifiedDir/opaqueDir1/newOpaqueFile1 && touch /unmodifiedDir/opaqueDir1/newOpaqueSubdir1/newOpaqueFile2 && rm -rf /modifyDir/opaqueDir2 && mkdir -p /modifyDir/opaqueDir2/newOpaqueSubdir2 && touch /modifyDir/opaqueDir2/newOpaqueFile3 && touch /modifyDir/opaqueDir2/newOpaqueSubdir2/newOpaqueFile4" did not complete successfully: exit code: 4294967295
        	            	github.com/moby/buildkit/util/stack.Enable
        	            		/src/util/stack/stack.go:77
        	            	github.com/moby/buildkit/util/grpcerrors.FromGRPC
        	            		/src/util/grpcerrors/grpcerrors.go:197
        	            	github.com/moby/buildkit/util/grpcerrors.UnaryClientInterceptor
        	            		/src/util/grpcerrors/intercept.go:41
        	            	google.golang.org/grpc.(*ClientConn).Invoke
        	            		/src/vendor/google.golang.org/grpc/call.go:35
        	            	github.com/moby/buildkit/api/services/control.(*controlClient).Solve
        	            		/src/api/services/control/control.pb.go:2208
        	            	github.com/moby/buildkit/client.(*Client).solve.func2
        	            		/src/client/solve.go:258
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		/src/vendor/golang.org/x/sync/errgroup/errgroup.go:75
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1594
        	            	failed to solve
        	            	github.com/moby/buildkit/client.(*Client).solve.func2
        	            		/src/client/solve.go:273
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		/src/vendor/golang.org/x/sync/errgroup/errgroup.go:75
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1594
        	Test:       	TestIntegration/TestDiffOpaqueDirsMerge/worker=oci-snapshotter-stargz

Start with client/mergediff_test.go around line 1252 and the linked CI job and logs, focusing on TestDiffOpaqueDirsMerge/worker=oci-snapshotter-stargz. Reproduce the failure and trace the unexpected error from the reported command. Done means the test's flakiness is explained and the named test no longer fails intermittently.