Add spellcheck smoke test

Exposes the spellcheck provider as window.spellChecker, which allows it to be
called from tests.

test/spellcheck_test.js

@@ -0,0 +1,10 @@
+/*
+ * vim: ts=4:sw=4:expandtab
+ */
+
+describe('spellChecker', function() {
+  it('should work', function() {
+    assert(window.spellChecker.spellCheck('correct'));
+    assert(!window.spellChecker.spellCheck('fhqwgads'));
+  });
+});