Test dispatch_after flakiness

[hjyamauchi]

This is on Windows. Running the tests repeatedly, I see that dispatch_after occasionally fails this way

      Start  7: dispatch_after
 7/22 Test  #7: dispatch_after ...................***Failed    9.70 sec

==================================================
[TEST] Dispatch After
[PID] 13484
==================================================


[BEGIN] can't finish faster than 5.5s
        Actual: 0
        Expected: <1007287300
[PASS] can't finish faster than 5.5s

[BEGIN] must finish faster than  6.5s
        Actual: 0
        Expected: <-7286800
[FAIL] must finish faster than  6.5s (S:\SourceCache\swift-corelibs-libdispatch\tests\dispatch_after.c:61)
        S:\SourceCache\swift-corelibs-libdispatch\tests\dispatch_after.c:61

[BEGIN] can't finish faster than 1.5s
        Actual: 0
        Expected: <510957700
[PASS] can't finish faster than 1.5s

[BEGIN] must finish faster than  2.5s
        Actual: 0
        Expected: <489042500
[PASS] must finish faster than  2.5s

[BEGIN] can't finish faster than 0s
        Actual: 0
        Expected: <7800
[PASS] can't finish faster than 0s

[BEGIN] must finish faster than .5s
        Actual: 0
        Expected: <499992500
[PASS] must finish faster than .5s

[BEGIN] Process exited
        Actual: 1
        Expected: 1
[PASS] Process exited
[PERF]  wall time: 9.670176
[PERF]  user time: 0.000000
[PERF]  system time: 0.000000
[PERF]  max working set size: 9670656
[PERF]  page faults: 2478

      Start  8: dispatch_timer
 8/22 Test  #8: dispatch_timer ...................   Passed    2.19 sec
      Start  9: dispatch_timer_short
 9/22 Test  #9: dispatch_timer_short .............   Passed    2.48 sec
      Start 10: dispatch_timer_timeout
10/22 Test #10: dispatch_timer_timeout ...........   Passed    6.19 sec
      Start 11: dispatch_sema
11/22 Test #11: dispatch_sema ....................   Passed    0.16 sec
      Start 12: dispatch_timer_bit31
12/22 Test #12: dispatch_timer_bit31 .............   Passed    3.16 sec
      Start 13: dispatch_timer_bit63
13/22 Test #13: dispatch_timer_bit63 .............   Passed    3.36 sec
      Start 14: dispatch_timer_set_time
14/22 Test #14: dispatch_timer_set_time ..........   Passed    2.67 sec
      Start 15: dispatch_data
15/22 Test #15: dispatch_data ....................   Passed    0.15 sec
      Start 16: dispatch_io_muxed
16/22 Test #16: dispatch_io_muxed ................   Passed    1.18 sec
      Start 17: dispatch_io_net
17/22 Test #17: dispatch_io_net ..................   Passed    0.32 sec
      Start 18: dispatch_io_pipe
18/22 Test #18: dispatch_io_pipe .................   Passed   12.42 sec
      Start 19: dispatch_io_pipe_close
19/22 Test #19: dispatch_io_pipe_close ...........   Passed    0.17 sec
      Start 20: dispatch_select
20/22 Test #20: dispatch_select ..................   Passed    0.19 sec
      Start 21: dispatch_c99
21/22 Test #21: dispatch_c99 .....................   Passed    0.17 sec
      Start 22: dispatch_plusplus
22/22 Test #22: dispatch_plusplus ................   Passed    0.16 sec

95% tests passed, 1 tests failed out of 22

Total Test time (real) =  51.74 sec

The following tests FAILED:
          7 - dispatch_after (Failed)
Errors while running CTest
exit