Improve tests and cleanup for PKI RevocationDistributionPoints

[ashcherbakov]

Scope:

• Only the tests for RevocationDistributionPoints (Add, Update, Delete) are in scope
• Two goals:
  • check test coverage
  • refactoring/simplification
• Both unit (pki/handler_tests.go) and integration tests

Details:

• add more CLI and gRPC/REST tests (see TBD in https://github.com/zigbee-alliance/distributed-compliance-ledger/blob/master/integration_tests/cli/pki-revocation-points.sh)
• make sure Unit Tests coverage is enough

• simplify the unit tests (, simplify initialisation, ):

  • simplify initialisation and test stricture by creating helper functions, for example

    // prepare
    add_vendor()
    add_root_cert(cert_path)
    
    // execution
    add_revocation_point()
    
    //check
    asserts... 

  • get rid of magic numbers (for example, make sure VID constant is issued instead of hard-coded values)
  • group similar tests as in TestHandler_UpdatePkiRevocationDistributionPoint_PAI_VIDPID
• improve error messages (all error text needs to be in errors.go's NewXXX methods, not error strings created on the fly).