Refactor server test setup code

[yuryu]

Many tests in open_savess_test.go has seven lines for setup if they need a store and a record to work with.

	ctx := context.Background()
	_, listener := getOpenSavesServer(ctx, t, "gcp")
	_, client := getTestClient(ctx, t, listener)
	store := &pb.Store{Key: uuid.NewString()}
	setupTestStore(ctx, t, client, store)
	record := &pb.Record{Key: uuid.NewString()}
	record = setupTestRecord(ctx, t, client, store.Key, record)

This can and should be shorter, for example,

1. create context
2. create client
3. create a default store and record

Also, tests use uuid as store and record keys to avoid conflicts, but something more predictable would be better for debugging. For example, "test name YYYY-MM-DD HH:mm:ss UUID" would let you know which entity corresponds to which test.