Fixes TSAN race by moving cleanup of platform into dtor.

- Before since it was in the TearDown, cleanup can be called before
  the device was gone which meant that async tasks could still be in
  flight.

Bug: dawn:1374
Change-Id: I4cd5a4e096a3fe13e230197a6a4d4d57daea31c8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/133480
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>

src/dawn/tests/DawnTest.cpp

@@ -739,6 +739,9 @@ DawnTestBase::~DawnTestBase() {
     // Check that all devices were destructed.
     EXPECT_EQ(gTestEnv->GetInstance()->GetDeviceCountForTesting(), 0u);
 
+    // Unsets the platform since we are cleaning the per-test platform up with the test case.
+    dawn::native::FromAPI(gTestEnv->GetInstance()->Get())->SetPlatformForTesting(nullptr);
+
     gCurrentTest = nullptr;
 }
 
@@ -1113,9 +1116,6 @@ void DawnTestBase::TearDown() {
         EXPECT_EQ(mLastWarningCount,
                   dawn::native::GetDeprecationWarningCountForTesting(device.Get()));
     }
-
-    // Unsets the platform since we are cleaning the per-test platform up with the test case.
-    dawn::native::FromAPI(gTestEnv->GetInstance()->Get())->SetPlatformForTesting(nullptr);
 }
 
 void DawnTestBase::DestroyDevice(wgpu::Device device) {