Reset the platform at MockPlatform in the test

This CL fixes the problem that the platform was set at MockPlatform's
constructor but was not reset.

Bug: v8:8527
Change-Id: I21c3b19320885b1b38999161db7cc1b8f15d798e
Reviewed-on: https://chromium-review.googlesource.com/c/1397821Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58574}

test/cctest/test-api.cc

@@ -25032,11 +25032,11 @@ namespace {
 
 class MockPlatform : public TestPlatform {
  public:
-  MockPlatform() {
+  MockPlatform() : old_platform_(i::V8::GetCurrentPlatform()) {
     // Now that it's completely constructed, make this the current platform.
     i::V8::SetPlatformForTesting(this);
   }
-  ~MockPlatform() override = default;
+  ~MockPlatform() override { i::V8::SetPlatformForTesting(old_platform_); }
 
   bool dump_without_crashing_called() const {
     return dump_without_crashing_called_;
@@ -25045,6 +25045,7 @@ class MockPlatform : public TestPlatform {
   void DumpWithoutCrashing() override { dump_without_crashing_called_ = true; }
 
  private:
+  v8::Platform* old_platform_;
   bool dump_without_crashing_called_ = false;
 };