chore: unflake ITAutogeneratedAdminClientTest (#3260)

Fixes #3239

Author: olavloite

google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITAutogeneratedAdminClientTest.java

@@ -266,9 +266,9 @@ private Database createAndUpdateDatabase(
 
   private String getCreateTableStatement() {
     if (dialect == DatabaseDialect.POSTGRESQL) {
-      return "CREATE TABLE T (" + "  \"K\"    VARCHAR PRIMARY KEY" + ")";
+      return "CREATE TABLE IF NOT EXISTS T (" + "  \"K\"    VARCHAR PRIMARY KEY" + ")";
     } else {
-      return "CREATE TABLE T (" + "  K    STRING(MAX)" + ") PRIMARY KEY (K)";
+      return "CREATE TABLE IF NOT EXISTS T (" + "  K    STRING(MAX)" + ") PRIMARY KEY (K)";
     }
   }