add acceptance tests for string value page rules

[jafowler]

➜ TF_ACC=1 go test ./internal/services/page_rule/ -run "^TestAccCloudflarePageRule_.*_String" -v -count=1
=== RUN   TestAccCloudflarePageRule_BypassCacheOnCookie_String
--- PASS: TestAccCloudflarePageRule_BypassCacheOnCookie_String (3.93s)
=== RUN   TestAccCloudflarePageRule_CacheLevel_String
--- PASS: TestAccCloudflarePageRule_CacheLevel_String (13.77s)
=== RUN   TestAccCloudflarePageRule_CacheOnCookie_String
--- PASS: TestAccCloudflarePageRule_CacheOnCookie_String (3.49s)
=== RUN   TestAccCloudflarePageRule_HostHeaderOverride_String
--- PASS: TestAccCloudflarePageRule_HostHeaderOverride_String (3.44s)
=== RUN   TestAccCloudflarePageRule_Polish_String
--- PASS: TestAccCloudflarePageRule_Polish_String (8.70s)
=== RUN   TestAccCloudflarePageRule_ResolveOverride_String
--- PASS: TestAccCloudflarePageRule_ResolveOverride_String (3.48s)
=== RUN   TestAccCloudflarePageRule_SSL_String
--- PASS: TestAccCloudflarePageRule_SSL_String (13.63s)
=== RUN   TestAccCloudflarePageRule_SecurityLevel_String
--- PASS: TestAccCloudflarePageRule_SecurityLevel_String (16.77s)
PASS
ok      github.com/cloudflare/terraform-provider-cloudflare/internal/services/page_rule 71.508s