test: Test StringUtils both with and without TextDecoder (#4405)

Since the TextDecoder fallback is only used on some devices, those
code paths were not tested on other platforms.  This makes the
StringUtil tests execute both with and without TextDecoder.

test/util/string_utils_unit.js

@@ -5,6 +5,29 @@
  */
 
 describe('StringUtils', () => {
+  describe('with TextDecoder', () => {
+    if (window.TextDecoder) {
+      defineStringUtilTests();
+    }
+  });
+
+  describe('without TextDecoder', () => {
+    let originalTextDecoder;
+
+    beforeAll(() => {
+      originalTextDecoder = window.TextDecoder;
+      window['TextDecoder'] = null;
+    });
+
+    afterAll(() => {
+      window.TextDecoder = originalTextDecoder;
+    });
+
+    defineStringUtilTests();
+  });
+});
+
+function defineStringUtilTests() {
   const StringUtils = shaka.util.StringUtils;
 
   it('parses fromUTF8', () => {
@@ -140,4 +163,4 @@ describe('StringUtils', () => {
     expect(StringUtils.fromUTF16(buffer, true).length)
         .toBe(buffer.byteLength / 2);
   });
-});
+}