ValidationTest: Make custom device creation go through a virtual method

Previously tests that needed to use a device that wasn't the default
device created a new one via a method that only supported customizing
extensions. Instead of adding a second function the device creation is
made completely customizable by querying the test's device via a
overridable method.

This heavily refactors the QuerySet tests to use the new methods
(previously they were using 3 different devices in the same fixture) but
loses a little bit of coverage of what happens when some query
operations are done with mixed devices.

This is required for a follow-up CL that adds tests for a device created
with the "disallow_unsafe_api" toggle that needs to be set on device
creation.

Bug: chromium:1138528

Change-Id: Ic2f5d876adca251b34ea594f70f344ac7669910e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/31442
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/tests/unittests/validation/ErrorScopeValidationTests.cpp

@@ -38,9 +38,10 @@ class ErrorScopeValidationTest : public ValidationTest {
     }
 
     void TearDown() override {
+        ValidationTest::TearDown();
+
         // Delete mocks so that expectations are checked
         mockDevicePopErrorScopeCallback = nullptr;
-        ValidationTest::TearDown();
     }
 };