[IMP] web: mock server: enhance default get

Before this commit, the mock server "_mockDefaultGet" method was a bit
naive:
- it accepted any 'default_' field coming from the 'kwargs.context'
  regardless of the existence of the field
- it accepted any value for these fields

Now the method is looks a bit more like its original server-side
counterpart:
- the context keys are added if they correspond to actual fields
- a pseudo validation removes many2one fields values not corresponding
  to any record

addons/web/static/tests/helpers/mock_server.js

@@ -482,27 +482,38 @@ var MockServer = Class.extend({
      * @private
      * @param {string} modelName
      * @param {array[]} args a list with a list of fields in the first position
-     * @param {Object} [kwargs]
+     * @param {Object} [kwargs={}]
      * @param {Object} [kwargs.context] the context to eventually read default
      *   values
      * @returns {Object}
      */
-    _mockDefaultGet: function (modelName, args, kwargs) {
-        var result = {};
-        var fields = args[0];
-        var model = this.data[modelName];
-        _.each(fields, function (name) {
-            var field = model.fields[name];
+    _mockDefaultGet: function (modelName, args, kwargs={}) {
+        const fields = args[0];
+        const model = this.data[modelName];
+        const result = {};
+        for (const fieldName of fields) {
+            const key = "default_" + fieldName;
+            if (kwargs.context && key in kwargs.context) {
+                result[fieldName] = kwargs.context[key];
+                continue;
+            }
+            const field = model.fields[fieldName];
             if ('default' in field) {
-                result[name] = field.default;
+                result[fieldName] = field.default;
+                continue;
             }
-        });
-        if (kwargs && kwargs.context)
-        _.each(kwargs.context, function (value, key) {
-            if ('default_' === key.slice(0, 8)) {
-                result[key.slice(8)] = value;
+        }
+        for (const fieldName in result) {
+            const field = model.fields[fieldName];
+            if (field.type === "many2one") {
+                const recordExists = this.data[field.relation].records.some(
+                    (r) => r.id === result[fieldName]
+                );
+                if (!recordExists) {
+                    delete result[fieldName];
+                }
             }
-        });
+        }
         return result;
     },
     /**