tests/resource/aws_spot_instance_request: Remove hardcoded instance types (#15161)
tests/resource/aws_spot_instance_request: Remove hardcoded instance types (#15161)

Reference: https://github.com/terraform-providers/terraform-provider-aws/issues/15160

Output from acceptance testing:

```
--- PASS: TestAccAWSSpotInstanceRequest_basic (292.33s)
--- PASS: TestAccAWSSpotInstanceRequest_getPasswordData (152.42s)
--- PASS: TestAccAWSSpotInstanceRequest_InterruptHibernate (299.56s)
--- PASS: TestAccAWSSpotInstanceRequest_InterruptStop (310.35s)
--- PASS: TestAccAWSSpotInstanceRequest_NetworkInterfaceAttributes (312.62s)
--- PASS: TestAccAWSSpotInstanceRequest_SubnetAndSGAndPublicIpAddress (333.97s)
--- PASS: TestAccAWSSpotInstanceRequest_tags (89.60s)
--- PASS: TestAccAWSSpotInstanceRequest_validUntil (322.83s)
--- PASS: TestAccAWSSpotInstanceRequest_vpc (311.49s)
--- PASS: TestAccAWSSpotInstanceRequest_withBlockDuration (282.54s)
--- PASS: TestAccAWSSpotInstanceRequest_withLaunchGroup (322.84s)
--- PASS: TestAccAWSSpotInstanceRequest_withoutSpotPrice (312.34s)
```
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws