docs(tests): rewrite CERTInext.Tests/TESTING.md to match the real test suite

The file documented a generic plugin template (X-API-Key auth, /api/v1/* endpoints,
AuthMode Basic/CertificateBased, cert-aaa-111) that bears no relation to the actual
tests. Rewrite it against the real project: HMAC-signed-body auth, the real CERTInext
endpoints (ValidateCredentials/GenerateOrderSSL/TrackOrder/GetOrderReport/GetCertificate/
RevokeOrder/GetDcv/VerifyDcv), AuthMode AccessKey/OAuth, and the real MockCertificateData
constants. (Documents 7 of the 11 test classes; the coverage/serial/redaction classes
can be added in a follow-up.)

Author: spbsoluble