Reduce clock skew in flaky duration tests

There are three changes here that should help reduce the
amount of duration test failures caused by clock skew.

First, we reported some skew when the test expected playback
to start immediately after loading the license. However,
with round-off, this could easily be more than 1 second. So
this does not warrent even a warning.

Second, the fake and real clocks were only synced after
computing how long to sleep. This is fixed by moving
SleepUntil to the TestSleep class and having it sync before
computing the delta and after doing the sleep.

Third, I am guessing that some failures due to unexpected
lenience were caused by the rental or playback clock being
started at the end of signing the license or the end of the
first decrypt instead of the beginning. We work around this
by recording how long these operations take, and then adding
this extra time at the end of the check for FailDecrypt.

Bug: 275003529
Bug: 279249646
Bug: 207500749
Merged from https://widevine-internal-review.googlesource.com/176070

Change-Id: I6a973565edfbebca53ee7f239b4b93f8f73d1e0a

libwvdrmengine/cdm/util/test/test_sleep.h

@@ -13,7 +13,9 @@ namespace wvutil {
 
 class TestSleep {
  public:
-  // The callback is called when the clock should be advanced.
+  // The callback is called when the test clock should be advanced. If the
+  // system uses a real clock, it is used to sync the real and test
+  // clock. Otherwise it is used to simulate sleep in the test clock.
   class CallBack {
    public:
     virtual void ElapseTime(int64_t milliseconds) = 0;
@@ -27,6 +29,9 @@ class TestSleep {
   // callback exists, this calls the callback.
   static void Sleep(unsigned int seconds);
 
+  // Like sleep, above, except it sleeps until the specified time.
+  static void SleepUntil(int64_t desired_time);
+
   // If we are using a real clock and a fake clock, then the real clock advances
   // a little while we are doing work, but the fake one only advances when we
   // sleep. This function advances the fake clock to be in sync with the real