Refactor test from async to sync

Reason for change: This test has been flaky on our CI due to its async
nature.

The expectation is for .build to construct TextEditor with the right
params.

We don't have to load the correct language package for this test. We can
load a dummy languageMode and assert that the correct grammar is set on
TextEditor

spec/text-editor-registry-spec.js

@@ -3,6 +3,7 @@ const TextEditor = require('../src/text-editor');
 const TextBuffer = require('text-buffer');
 const { Point, Range } = TextBuffer;
 const dedent = require('dedent');
+const NullGrammar = require('../src/null-grammar');
 
 describe('TextEditorRegistry', function() {
   let registry, editor, initialPackageActivation;
@@ -69,16 +70,24 @@ describe('TextEditorRegistry', function() {
   });
 
   describe('.build', function() {
-    it('constructs a TextEditor with the right parameters based on its path and text', async function() {
-      await atom.packages.activatePackage('language-javascript');
-      await atom.packages.activatePackage('language-c');
-
+    it('constructs a TextEditor with the right parameters based on its path and text', function() {
       atom.config.set('editor.tabLength', 8, { scope: '.source.js' });
 
+      const languageMode = {
+        grammar: NullGrammar,
+        onDidChangeHighlighting: jasmine.createSpy()
+      };
+
+      const buffer = new TextBuffer({ filePath: 'test.js' });
+      buffer.setLanguageMode(languageMode);
+
       const editor = registry.build({
-        buffer: new TextBuffer({ filePath: 'test.js' })
+        buffer
       });
+
       expect(editor.getTabLength()).toBe(8);
+      expect(editor.getGrammar()).toEqual(NullGrammar);
+      expect(languageMode.onDidChangeHighlighting.calls.length).toBe(1);
     });
   });