fix(zone_cache_variants): use MarshalRoot for updates

[sjparkinson]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

• Fix update requests for zone_cache_variants resource by not including removed variants in the request body.
• Add a new acceptance test to cover the behaviour.

Acceptance test run results

• I have added or updated acceptance tests for my changes
• I have run acceptance tests for my changes and included the results below

Steps to run acceptance tests

Test output

Additional context & links

Fixes #6700.

I used a similar approach to the fix in the custom_hostname resource¹.

Footnotes

1. https://github.com/cloudflare/terraform-provider-cloudflare/blob/b44cf8a64c5152a76094cc0844e6f02da8000556/internal/services/custom_hostname/model.go#L35-L44 ↩

[sjparkinson]

Thanks for running the tests, I'll dig into the failure.

=== RUN   TestAccCloudflareZoneCacheVariants_OneExt
    resource_test.go:66: Acceptance tests skipped unless env 'TF_ACC' set
--- SKIP: TestAccCloudflareZoneCacheVariants_OneExt (0.00s)
=== RUN   TestAccCloudflareZoneCacheVariants_AllExt
    resource_test.go:99: Acceptance tests skipped unless env 'TF_ACC' set
--- SKIP: TestAccCloudflareZoneCacheVariants_AllExt (0.00s)
=== RUN   TestAccCloudflareZoneCacheVariants_RemoveSingleVariantKey
    resource_test.go:150: Test validation error: TestStep 2/2 validation error: TestStep missing Config or ConfigDirectory or ConfigFile or ImportState or RefreshState
--- FAIL: TestAccCloudflareZoneCacheVariants_RemoveSingleVariantKey (0.00s)
=== RUN   TestAccUpgradeZoneCacheVariants_FromPublishedV5
    resource_test.go:199: Acceptance tests skipped unless env 'TF_ACC' set
--- SKIP: TestAccUpgradeZoneCacheVariants_FromPublishedV5 (0.00s)
=== CONT  TestZoneCacheVariantsModelSchemaParity
--- PASS: TestZoneCacheVariantsModelSchemaParity (0.00s)
=== CONT  TestZoneCacheVariantsDataSourceModelSchemaParity
--- PASS: TestZoneCacheVariantsDataSourceModelSchemaParity (0.00s)
FAIL
FAIL	github.com/cloudflare/terraform-provider-cloudflare/internal/services/zone_cache_variants	0.024s

Ah missed including the new fixture! f180900 should resolve it.