test(image_variant): add basic acceptance tests

[musa-cf]

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

Changes being requested

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

TF_ACC=1 go test ./internal/services/image_variant -run 'TestAcc' -count=1 -v

Test output

=== RUN   TestAccCloudflareImageVariantDataSource_Basic
=== PAUSE TestAccCloudflareImageVariantDataSource_Basic
=== RUN   TestAccCloudflareImageVariant_Basic
=== PAUSE TestAccCloudflareImageVariant_Basic
=== CONT  TestAccCloudflareImageVariantDataSource_Basic
=== CONT  TestAccCloudflareImageVariant_Basic
--- PASS: TestAccCloudflareImageVariant_Basic (4.59s)
--- PASS: TestAccCloudflareImageVariantDataSource_Basic (6.07s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/internal/services/image_variant	8.018s

Additional context & links