test(spanner): make sequence sample integration tests resilient to retries (googleapis#13183)

Updates `SequenceSampleIT` and its archived counterpart to verify that
the count of per-row console output messages is a positive multiple of
3. This prevents flakiness caused by Cloud Spanner read-write
transaction retries while keeping the public sample snippet code
pristine and minimal for documentation readers.

Author: sakthivelmanii

java-spanner/samples/snippets/src/test/java/com/example/spanner/SequenceSampleIT.java

@@ -93,7 +93,8 @@ public void createSequence() throws Exception {
         out.contains(
             "Created Seq sequence and Customers table, where the key column "
                 + "CustomerId uses the sequence as a default value"));
-    assertEquals(out.split("Inserted customer record with CustomerId", -1).length - 1, 3);
+    int insertCount = out.split("Inserted customer record with CustomerId", -1).length - 1;
+    assertTrue(insertCount > 0 && insertCount % 3 == 0);
     assertTrue(out.contains("Number of customer records inserted is: 3"));
   }
 
@@ -115,7 +116,8 @@ public void alterSequence() throws Exception {
     }
     assertTrue(
         out.contains("Altered Seq sequence to skip an inclusive range between 1000 and 5000000"));
-    assertEquals(out.split("Inserted customer record with CustomerId", -1).length - 1, 3);
+    int insertCount = out.split("Inserted customer record with CustomerId", -1).length - 1;
+    assertTrue(insertCount > 0 && insertCount % 3 == 0);
     assertTrue(out.contains("Number of customer records inserted is: 3"));
   }
 

java-spanner/samples/snippets/src/test/java/com/example/spanner/admin/archived/SequenceSampleIT.java

@@ -96,7 +96,8 @@ public void createSequence() throws Exception {
         out.contains(
             "Created Seq sequence and Customers table, where its key column "
                 + "CustomerId uses the sequence as a default value"));
-    assertEquals(out.split("Inserted customer record with CustomerId", -1).length - 1, 3);
+    int insertCount = out.split("Inserted customer record with CustomerId", -1).length - 1;
+    assertTrue(insertCount > 0 && insertCount % 3 == 0);
     assertTrue(out.contains("Number of customer records inserted is: 3"));
   }
 
@@ -118,7 +119,8 @@ public void alterSequence() throws Exception {
     }
     assertTrue(
         out.contains("Altered Seq sequence to skip an inclusive range between 1000 and 5000000"));
-    assertEquals(out.split("Inserted customer record with CustomerId", -1).length - 1, 3);
+    int insertCount = out.split("Inserted customer record with CustomerId", -1).length - 1;
+    assertTrue(insertCount > 0 && insertCount % 3 == 0);
     assertTrue(out.contains("Number of customer records inserted is: 3"));
   }