test: Fix prefetch test fake pollution (#6285)

When overwriting any part of the library or platform in a test, we must
restore it afterward.

This fixes two test suites that overwrote SegmentPrefetch with a test
fake, but never restored the original.

This fixes failures in unrelated preload tests running in uncompiled
mode.

Closes #6275

test/media/streaming_engine_unit.js

@@ -3996,6 +3996,12 @@ describe('StreamingEngine', () => {
   }
 
   describe('prefetch segments', () => {
+    let originalPrefetch;
+
+    beforeAll(() => {
+      originalPrefetch = shaka.media.SegmentPrefetch;
+    });
+
     beforeEach(() => {
       shaka.media.SegmentPrefetch = Util.spyFunc(
           jasmine.createSpy('SegmentPrefetch')
@@ -4013,6 +4019,10 @@ describe('StreamingEngine', () => {
       streamingEngine.configure(config);
     });
 
+    afterAll(() => {
+      shaka.media.SegmentPrefetch = originalPrefetch;
+    });
+
     it('should use prefetched segment without fetching again', async () => {
       streamingEngine.switchVariant(variant);
       await streamingEngine.start();
@@ -4080,6 +4090,12 @@ describe('StreamingEngine', () => {
     let altVariant;
     let segmentPrefetchesCreated;
 
+    let originalPrefetch;
+
+    beforeAll(() => {
+      originalPrefetch = shaka.media.SegmentPrefetch;
+    });
+
     beforeEach(() => {
       segmentPrefetchesCreated = 0;
       shaka.media.SegmentPrefetch = Util.spyFunc(
@@ -4101,6 +4117,10 @@ describe('StreamingEngine', () => {
       streamingEngine.configure(config);
     });
 
+    afterAll(() => {
+      shaka.media.SegmentPrefetch = originalPrefetch;
+    });
+
     it('should prefetch all audio variants for the language', async () => {
       const segmentType = shaka.net.NetworkingEngine.RequestType.SEGMENT;
       const context = {