[ThreadPool]: Fix JoinJobTaskSource

Problem: EXPECT_EQ(1U, task_source->HasOneRef()) fails because threads don't
release ref right away.
Fix: use JoinForTesting

Bug: 1011502
Change-Id: I4f5c7da2d3cf66cd6635a97c4b99acba866c2db3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1845232
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703689}

base/task/thread_pool/thread_group_unittest.cc

@@ -801,7 +801,10 @@ TEST_P(ThreadGroupTest, JoinJobTaskSource) {
   job_handle.Join();
   // All worker tasks should complete before Join() returns.
   EXPECT_EQ(0U, job_task->GetMaxConcurrency());
+  thread_group_->JoinForTesting();
   EXPECT_EQ(1U, task_source->HasOneRef());
+  // Prevent TearDown() from calling JoinForTesting() again.
+  thread_group_ = nullptr;
 }
 
 // Verify that the maximum number of BEST_EFFORT tasks that can run concurrently