Backport some tests in /components/consent_auditor to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/components/consent_auditor.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=vitaliii@chromium.org

Bug: 891670
Change-Id: Ia30be54ecda2737d30a4eee109d023673342361d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786689
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: vitaliii <vitaliii@chromium.org>
Commit-Queue: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694225}

components/consent_auditor/consent_sync_bridge_impl_unittest.cc

@@ -164,7 +164,7 @@ class ConsentSyncBridgeImplTest : public testing::Test {
   }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   testing::NiceMock<MockModelTypeChangeProcessor> mock_processor_;
   std::unique_ptr<ConsentSyncBridgeImpl> bridge_;
 };