internal/grpcsync: move CallbackSerializer from xdsclient/internal to here

[my4-dev]

Move CallbackSerializer from xdsclient/internal to grpcsync package
Please refer to this conversation in #6036 .

RELEASE NOTES: none

[easwars]

Is this const being anywhere?

[my4-dev]

This const was originally defined in event_test.go at xdsclient package and it is used in callback_serializer_test.go.
Since const values with the same meaning were not defined, I've copied it here and resolved a compile error.

[easwars]

There is no file named event_test.go in the xdsclient package. Also, I checked callback_serializer_test.go and there is no usage of defaultTestWatchExpiryTimeout. Can you please delete it. Thanks.

[my4-dev]

The test entry point like the following is defined in event_test.go in the grpcsync package.
So callback_serializer_test.go depends on event_test.go. Should I revise this file name or do anything else?

type s struct {
	grpctest.Tester
}

func Test(t *testing.T) {
	grpctest.RunSubTests(t, s{})
}

Also, I didn't notice that there is no usage of defaultTestWatchExpiryTimeout. I've deleted this definition from callback_serializer_test.go.

[easwars]

Also, I didn't notice that there is no usage of defaultTestWatchExpiryTimeout. I've deleted this definition from callback_serializer_test.go.

Yes, that's all I wanted. Thanks.

[my4-dev]

Again, one test pipeline failed. A test time out seems to cause this fail.
I've implemented the following commands in my local machine and they've succeeded.

VET_SKIP_PROTO=1 ./vet.sh
go test -cpu 1,4 -timeout 7m ./...
go test -race -cpu 1,4 -timeout 7m ./...

May it be a flaky test?
I'm not sure why this fail occured.

[dfawley]

This is a known flake: #6075