[clangd][tests] Assert for idleness of scheduler

[kadircet]

Summary

We could fail going idle in 5 seconds and get spurious errors
afterwards. See https://github.com/llvm/llvm-project/issues/64964.

clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp

TEST_F(TUSchedulerTests, PublishWithStalePreamble) {
   PI.Contents = "#define BAR\n" + PI.Version;
   ASSERT_THAT(BlockForDiags(PI), testing::Pair("1", "2"));
 
   PI.Version = "3";
   PI.Contents = "#define FOO\n" + PI.Version;
   ASSERT_THAT(BlockForDiags(PI), testing::Pair("1", "3"));
 
   UnblockPreamble.notify();
-  S.blockUntilIdle(timeoutSeconds(5));
+  ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(5)));
 
   // Make sure that we have eventual consistency.
   EXPECT_THAT(Collector.diagVersions().back(), Pair(PI.Version, PI.Version));
 
   // Check that WantDiagnostics::No doesn't emit any diags.
   PI.Version = "4";
   PI.Contents = "#define FOO\n" + PI.Version;
   S.update(File, PI, WantDiagnostics::No);
-  S.blockUntilIdle(timeoutSeconds(5));
+  ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(5)));
   EXPECT_THAT(Collector.diagVersions().back(), Pair("3", "3"));
 }
 
 // If a header file is missing from the CDB (or inferred using heuristics), and
 // it's included by another open file, then we parse it using that files flags.
 TEST_F(TUSchedulerTests, IncluderCache) {
   static std::string Main = testPath("main.cpp"), Main2 = testPath("main2.cpp"),
                      Main3 = testPath("main3.cpp"),