zjsunit: Add with_field helper.

This just lets us temporarily assign a value
to a field.

Differences with the "override" scheme:

    * override only works on globals
    * override (when passed in via run_test) will
      just automatically clean up at the end of
      the function

.eslintrc.json

@@ -348,6 +348,7 @@
                 "document": false,
                 "set_global": false,
                 "window": false,
+                "with_field": false,
                 "zrequire": false
             },
             "rules": {

frontend_tests/node_tests/markdown.js

@@ -758,12 +758,9 @@ run_test("missing unicode emojis", () => {
         // return undefined
     }
 
-    const actual_get_emoji_name = emoji.get_emoji_name;
-    emoji.get_emoji_name = fake_get_emoji_name;
-
+    with_field(emoji, "get_emoji_name", fake_get_emoji_name, () => {
         markdown.apply_markdown(message);
-    assert.equal(message.content, "<p>\u{1f6b2}</p>");
-
-    // restore get_emoji_name for any future tests that follow
-    emoji.get_emoji_name = actual_get_emoji_name;
+    });
+
+    assert.equal(message.content, "<p>\u{1f6b2}</p>");
 });

frontend_tests/zjsunit/index.js

@@ -37,6 +37,7 @@ if (files.length === 0) {
 }
 
 // Set up our namespace helpers.
+global.with_field = namespace.with_field;
 global.set_global = namespace.set_global;
 global.patch_builtin = namespace.set_global;
 global.zrequire = namespace.zrequire;

frontend_tests/zjsunit/namespace.js

@@ -69,6 +69,13 @@ exports.stub_out_jquery = function () {
     $.now = function () {};
 };
 
+exports.with_field = function (obj, field, val, f) {
+    const old_val = obj[field];
+    obj[field] = val;
+    f();
+    obj[field] = old_val;
+};
+
 exports.with_overrides = function (test_function) {
     // This function calls test_function() and passes in
     // a way to override the namespace temporarily.