test(spanner): deflake R/W transaction test with in-memory harness test for omni (#13514)

Author: rahul2393

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/LocationAwareSharedBackendReplicaHarnessTest.java

@@ -398,7 +398,11 @@ public void readWriteTransactionAbortedCommitUsesReadAffinityReplicaForBypassTra
               });
 
       assertEquals(2, attempts.get());
-      assertEquals(1, firstReplicaIndex.get());
+      assertNotEquals(
+          "Expected read-write transaction read to route to a bypass replica.\n"
+              + routingDiagnostics(harness),
+          -1,
+          firstReplicaIndex.get());
       int secondReplicaIndex = 1 - firstReplicaIndex.get();
       assertEquals(
           2,