[lldb] Make MainLoopTest::CallbackWithTimeout slightly more resilient (llvm#162197)

Compute the start time *before* registering the callback, rather than
after, to avoid the possibility of a small race.

The following scenario illustrates the problem.

1. The callback is registered with a 2 second timeout at t=0ms.
2. We compute the start time after registering the callback. For the
sake of argument, let's say it took 5ms to return from registering the
callback and computing the current time. Start=5ms.
3. The callback fires after exactly 2 seconds, or t=2000ms.
4. We compute the difference between start and now. If it took less than
5ms to compute, then we end up with a difference that's less than 2000ms
and the test fails. Let's say it took 3ms this time, then
2003ms-5ms=1998ms < 2000ms.

The actual values in the example above are arbitrary. All that matters
is that it took longer to compute the start time than the end time. My
theory is that this explains why this test is flaky when running under
ASan in CI (which has unpredictable timing).

rdar://160956999
(cherry picked from commit 605e2d1)

Author: JDevlieghere

lldb/unittests/Host/MainLoopTest.cpp

@@ -421,9 +421,9 @@ TEST_F(MainLoopTest, ManyPendingCallbacks) {
 
 TEST_F(MainLoopTest, CallbackWithTimeout) {
   MainLoop loop;
+  auto start = std::chrono::steady_clock::now();
   loop.AddCallback([](MainLoopBase &loop) { loop.RequestTermination(); },
                    std::chrono::seconds(2));
-  auto start = std::chrono::steady_clock::now();
   ASSERT_THAT_ERROR(loop.Run().takeError(), llvm::Succeeded());
   EXPECT_GE(std::chrono::steady_clock::now() - start, std::chrono::seconds(2));
 }