tests/provider: Update testAccPreCheck to also note AWS_PROFILE environment variable usage
tests/provider: Update testAccPreCheck to also note AWS_PROFILE environment variable usage

Output from acceptance testing:

```console
$ env | grep AWS_
$ TF_ACC=1 go test ./aws -v -timeout 120m -parallel 20 -run='TestAccAWSAvailabilityZones_basic'
...
--- FAIL: TestAccAWSAvailabilityZones_basic (0.00s)
    provider_test.go:70: AWS_ACCESS_KEY_ID or AWS_PROFILE must be set for acceptance tests

$ export AWS_PROFILE=tf-acc
$ TF_ACC=1 go test ./aws -v -timeout 120m -parallel 20 -run='TestAccAWSAvailabilityZones_basic'
...
--- PASS: TestAccAWSAvailabilityZones_basic (9.53s)

$ export AWS_ACCESS_KEY_ID=$(aws configure get aws_access_key_id)
$ export AWS_SECRET_ACCESS_KEY=$(aws configure get aws_secret_access_key)
$ unset AWS_PROFILE
$ TF_ACC=1 go test ./aws -v -timeout 120m -parallel 20 -run='TestAccAWSAvailabilityZones_basic'
...
--- PASS: TestAccAWSAvailabilityZones_basic (9.49s)
```

kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws