Add acceptance test for ngwaf product enablement updates

[philippschulte]

Change summary

This PR adds an acceptance test to verify that configuration changes to the ngwaf block within the product_enablement attribute of the fastly_service_vcl resource are correctly applied and do not result in persistent diffs.

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=== RUN   TestAccFastlyServiceVCLProductEnablement_basic
=== PAUSE TestAccFastlyServiceVCLProductEnablement_basic
=== CONT  TestAccFastlyServiceVCLProductEnablement_basic
--- PASS: TestAccFastlyServiceVCLProductEnablement_basic (16.11s)

=== RUN   TestAccFastlyServiceVCLProductEnablement_ngwafUpdate
=== PAUSE TestAccFastlyServiceVCLProductEnablement_ngwafUpdate
=== CONT  TestAccFastlyServiceVCLProductEnablement_ngwafUpdate
--- PASS: TestAccFastlyServiceVCLProductEnablement_ngwafUpdate (46.63s)

[rcaril]

LGTM

[kpfleming]

Yep, this covers the necessary changes. I could have sworn that NGWAF had a mode attribute like DDoS Protection does but I don't see it in the API reference.