change to use correct project Id

Author: ldetmer

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/it/ITMutableCredentialsTest.java

@@ -75,7 +75,7 @@ public void testMutableCredentialsUpdateAuthorizationForRunningClient() throws I
         DatabaseAdminClient databaseAdminClient = spanner.createDatabaseAdminClient()) {
       String dbName =
           DatabaseName.of(
-                  GceTestEnvConfig.GCE_PROJECT_ID,
+                  getTestEnv().getTestHelper().getInstanceId().getProject(),
                   getTestEnv().getTestHelper().getInstanceId().getInstance(),
                   "TEST")
               .toString();