Backport some tests in /components/ntp_snippets 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/ntp_snippets.

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

This CL was uploaded by git cl split.

R=noyau@chromium.org

Bug: 891670
Change-Id: Ib3206c1a7c5241a2f8f6d515f7f321be5058519f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787205
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Eric Noyau <noyau@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695326}

components/ntp_snippets/content_suggestions_service_unittest.cc

@@ -259,7 +259,7 @@ TEST_F(ContentSuggestionsServiceTest, ShouldRedirectFetchSuggestionImage) {
 
 TEST_F(ContentSuggestionsServiceTest,
        ShouldCallbackEmptyImageForUnavailableProvider) {
-  base::test::TaskEnvironment task_environment;
+  base::test::SingleThreadTaskEnvironment task_environment;
 
   base::RunLoop run_loop;
   // Assuming there will never be a category with the id below.