tests/resource/aws_subnet: Fix errant resource name in TestAccAWSSubnet_availabilityZoneId test (#15162)
tests/resource/aws_subnet: Fix errant resource name in TestAccAWSSubnet_availabilityZoneId test (#15162)

Previously:

```
=== CONT  TestAccAWSSubnet_availabilityZoneId
TestAccAWSSubnet_availabilityZoneId: resource_aws_subnet_test.go:333: Check 3/3 error: Not found: data.aws_availability_zones.test in [root]
--- FAIL: TestAccAWSSubnet_availabilityZoneId (10.37s)
```

Output from acceptance testing:

```
--- PASS: TestAccAWSSubnet_availabilityZoneId (27.83s)
```
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws