Fix multiple device leaks in dawn_end2end_tests and dawn_unittests

Adds ForTesting APIs to the instance to track the number of devices.

Bug: dawn:1164
Change-Id: Ib743afb1e86ef16740d49613f43f9e2f009232bc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/90583
Reviewed-by: Loko Kung <lokokung@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

include/dawn/native/DawnNative.h

@@ -170,6 +170,8 @@ class DAWN_NATIVE_EXPORT Instance {
     // TODO(dawn:1374) Deprecate this once it is passed via the descriptor.
     void SetPlatform(dawn::platform::Platform* platform);
 
+    uint64_t GetDeviceCountForTesting() const;
+
     // Returns the underlying WGPUInstance object.
     WGPUInstance Get() const;