[UnitTests][Threading] Tweak ConditionVariable test for Windows.

Windows' behaviour wrt `Sleep` family functions can be odd.  Apparently,
if you specify a time lower than a system tick, they can return early.
This makes the test flaky when it shouldn't be.  To fix it, we use the
Multimedia functions to adjust the system tick count as low as we can
get it.

rdar://100236038

Author: al45tair

unittests/Threading/ConditionVariable.cpp

@@ -19,13 +19,51 @@
 #include "ThreadingHelpers.h"
 #include "LockingHelpers.h"
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
 using namespace swift;
 
 template <typename Body>
 std::chrono::duration<double> measureDuration(Body body) {
+#ifdef _WIN32
+  using namespace std::chrono_literals;
+
+  /* On Windows, we use SleepConditionVariableSRW() to implement
+     ConditionVariable.  The way Windows implements the SleepXXX() functions,
+     if you specify a timeout smaller than the current system tick rate,
+     the function may return early.  See
+
+     https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-sleep
+
+     This causes flakiness in this test, which is undesirable.
+
+     To combat this, we adjust the system tick rate temporarily here.
+
+     Even *after* doing that, measurement suggests that Windows can return
+     early by an amount not exceeding a system tick. */
+  TIMECAPS tc;
+  UINT uTimerRes;
+
+  ASSERT_NE(timeGetDevCaps(&tc, sizeof(TIMECAPS)), TIMERR_NOERROR);
+
+  uTimerRes = min(max(tc.wPeriodMin, 1/*ms*/), tc.wPeriodMax);
+  timeBeginPeriod(uTimerRes);
+#endif
+
   auto start = std::chrono::steady_clock::now();
   body();
-  return std::chrono::steady_clock::now() - start;
+  std::chrono::duration<double> elapsed = std::chrono::steady_clock::now() - start;
+
+#ifdef _WIN32
+  timeEndPeriod(uTimerRes);
+
+  // Lie about the elapsed time to make the test more robust
+  elapsed += 0.001s;
+#endif
+
+  return elapsed;
 }
 
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