Add unit test for Mailbox/clear method

Author: dab246

lib/jmap/core/error/set_error.dart

@@ -17,6 +17,7 @@ class SetError with EquatableMixin {
   static final invalidProperties = ErrorType("invalidProperties");
   static final invalidArguments = ErrorType("invalidArguments");
   static final singleton = ErrorType("singleton");
+  static final serverFail = ErrorType("serverFail");
 
   final ErrorType type;
   final String? description;
@@ -40,5 +41,6 @@ class SetError with EquatableMixin {
     SetError.willDestroy,
     SetError.invalidProperties,
     SetError.singleton,
+    SetError.serverFail,
   };
 }