Backport some tests in /components/captive_portal 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/captive_portal.

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

This CL was uploaded by git cl split.

R=mmenke@chromium.org

Bug: 891670
Change-Id: I1125b34060d286987b18944e071148b8b7dacb84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787097
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693711}

components/captive_portal/captive_portal_detector_unittest.cc

@@ -114,7 +114,7 @@ class CaptivePortalDetectorTest : public testing::Test,
   }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<CaptivePortalDetector> detector_;
 };