Address dependency issues in TestAccFirestoreField_* tests

[modular-magician]

Closes hashicorp/terraform-provider-google#17392

Adding an additional delay in TestAccFirestoreField_* tests to address an 100% error occurring since late Jan:

------- Stdout: -------
=== RUN   TestAccFirestoreField_firestoreFieldUpdateAddIndexExample
=== PAUSE TestAccFirestoreField_firestoreFieldUpdateAddIndexExample
=== CONT  TestAccFirestoreField_firestoreFieldUpdateAddIndexExample
    testing_new.go:84: Error running post-test destroy, there may be dangling resources: googleapi: Error 403: Cloud Firestore API has not been used in project tf-test3abifux6ev before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/firestore.googleapis.com/overview?project=tf-test3abifux6ev then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.
        Details:
        [
          {
            "@type": "type.googleapis.com/google.rpc.Help",
            "links": [
              {
                "description": "Google developers console API activation",
                "url": "https://console.developers.google.com/apis/api/firestore.googleapis.com/overview?project=tf-test3abifux6ev
              }
            ]
          },
          {
            "@type": "type.googleapis.com/google.rpc.ErrorInfo",
            "domain": "googleapis.com",
            "metadata": {
              "consumer": "projects/tf-test3abifux6ev",
              "service": "firestore.googleapis.com"
            },
            "reason": "SERVICE_DISABLED"
          }
        ]
--- FAIL: TestAccFirestoreField_firestoreFieldUpdateAddIndexExample (1163.75s)
FAIL

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Derived from GoogleCloudPlatform/magic-modules#9957