Add emulation of transaction retries to memory impl of RunInTransaction.

service/datastore/testable.go

Index: service/datastore/testable.go
diff --git a/service/datastore/testable.go b/service/datastore/testable.go
index 5887595f742b02b755476f2cddec4c73793d5cbe..14648dc124b5c0997f18f424765888e4fc6d7a6e 100644
--- a/service/datastore/testable.go
+++ b/service/datastore/testable.go
@@ -33,4 +33,9 @@ type Testable interface {
 	// But depending on the implementation it may implemented with an atomic
 	// operation.
 	CatchupIndexes()
+
+	// SetTransactionRetryCount set how many times RunInTransaction will retry
+	// transaction body pretending transaction conflicts happens. 0 (default)
+	// means commit succeeds on the first attempt (no retries).
+	SetTransactionRetryCount(int)
 }