Bug 1575823 - Enable support for l10n.getString in tests r=Ola

- This creates a mock for fluent-l10n module, so we can use `l10n.getString()` in our code and test for it.
- This patch also removes unused files `test/fixtures/l10n.js` and `test/fixtures/PluralForm.js`

In order to double check the mock works, these two lines can be added to any test:

```
const { l10n } = require("devtools/client/application/src/modules/l10n");
expect(l10n.getString("foo")).toBe("foo");
```

Differential Revision: https://phabricator.services.mozilla.com/D43050

--HG--
rename : devtools/client/application/test/components/fixtures/l10n.js => devtools/client/application/test/components/fixtures/fluent-l10n.js
extra : moz-landing-system : lando

devtools/client/application/test/components/fixtures/PluralForm.js

@@ -1,11 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
-
-"use strict";
-
-module.exports.PluralForm = {
-  get(num, str) {
-    return str;
-  },
-};

devtools/client/application/test/components/fixtures/fluent-l10n.js

@@ -0,0 +1,23 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * Mock for devtools/client/shared/modules/fluent-l10n/fluent-l10n
+ */
+class FluentL10n {
+  async init() {}
+
+  getBundles() {
+    return [];
+  }
+
+  getString(id, args) {
+    return args ? `${id}__${JSON.stringify(args)}` : id;
+  }
+}
+
+// Export the class
+exports.FluentL10n = FluentL10n;

devtools/client/application/test/components/fixtures/l10n.js

@@ -1,23 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
-
-"use strict";
-
-// @TODO Load the actual strings from instead.
-class L10N {
-  getStr(str) {
-    switch (str) {
-      default:
-        return str;
-    }
-  }
-
-  getFormatStr(str) {
-    return this.getStr(str);
-  }
-}
-
-module.exports = {
-  L10N: new L10N(),
-};

devtools/client/application/test/components/jest.config.js

@@ -9,11 +9,10 @@ module.exports = {
   verbose: true,
   moduleNameMapper: {
     // Custom name mappers for modules that require m-c specific API.
-    "^../utils/l10n": `${__dirname}/fixtures/l10n`,
     "^devtools/client/shared/link": `${__dirname}/fixtures/stub`,
-    "^devtools/shared/plural-form": `${__dirname}/fixtures/plural-form`,
     "^chrome": `${__dirname}/fixtures/Chrome`,
     "^Services": `${__dirname}/fixtures/Services`,
+    "^devtools/client/shared/fluent-l10n/fluent-l10n$": `${__dirname}/fixtures/fluent-l10n`,
     "^devtools/client/shared/unicode-url": `${__dirname}/fixtures/unicode-url`,
     // Map all require("devtools/...") to the real devtools root.
     "^devtools\\/(.*)": `${__dirname}/../../../../$1`,