Fix race in treapstore test.

common/data/treapstore/store_test.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package treapstore
 
 import (
         "fmt"
         "strconv"
         "strings"
@@ skipping 242 matching lines @@
                         go func() {
                                 doWrite(strconv.Itoa(i))
                                 close(writeDoneC)
                         }()
                         // The first round has to actually create the Collection.
                         go func() {
                                 readDoneC <- doReads()
                         }()
 
                         <-writeDoneC
-                        So(<-readDoneC, ShouldBeGreaterThan, 0)
+
+                        check := ShouldBeGreaterThan
+                        if i == 0 {
+                                // The first time around, we *could* read before anything has been
+                                // written. Every other time, something from the previous round will
+                                // have been written.
+                                check = ShouldBeGreaterThanOrEqualTo
+                        }
+                        So(<-readDoneC, check, 0)
                         snaps = append(snaps, head.Snapshot())
                 }
         })
 }