D9822: Add benchmarks for KDirWatch

[Milian Wolff]

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REVISION SUMMARY
  All benchmarks create a relatively large directory tree and then
  watch that recursively. On my system, the timings are:
  
  RESULT : KDirWatch_UnitTest::benchCreateTree():
  
    34.2 msecs per iteration (total: 342, iterations: 10)
  
  PASS   : KDirWatch_UnitTest::benchCreateWatcher()
  RESULT : KDirWatch_UnitTest::benchCreateWatcher():
  
    13.6 msecs per iteration (total: 137, iterations: 10)
  
  PASS   : KDirWatch_UnitTest::benchNotifyWatcher()
  RESULT : KDirWatch_UnitTest::benchNotifyWatcher():
  
    113.7 msecs per iteration (total: 1,138, iterations: 10)
  
  Note that this time includes wait/sleep time and the latency
  induced by the kernel for the inotify events to arrive. Measuring
  on-CPU time can be done by passing `-perf` to the benchmarks:
  
  PASS   : KDirWatch_UnitTest::benchCreateTree():
  RESULT : KDirWatch_UnitTest::benchCreateTree():
  
    165,910,830.4 CPU cycles per iteration (total: 1,659,108,304, iterations: 10)
  
  PASS   : KDirWatch_UnitTest::benchCreateWatcher()
  RESULT : KDirWatch_UnitTest::benchCreateWatcher():
  
    64,014,469.8 CPU cycles per iteration (total: 640,144,699, iterations: 10)
  
  PASS   : KDirWatch_UnitTest::benchNotifyWatcher()
  RESULT : KDirWatch_UnitTest::benchNotifyWatcher():
  
    316,077,005.5 CPU cycles per iteration (total: 3,160,770,055, iterations: 10)
  
  The first benchmark measures the time it takes to create the large
  directory. See below for why this is useful information.
  
  The second measures how long it takes to add the root dir
  and its contents recursively.
  
  The third one touches all files in the tree and measure how long it
  takes until all dirty signals arrive. If you subtract that measurement
  from the third benchmark, you arrive at the overhead imposed by
  KDirWatch. At this point, it's quite high!

REPOSITORY
  R244 KCoreAddons

REVISION DETAIL
  https://phabricator.kde.org/D9822

AFFECTED FILES
  autotests/kdirwatch_unittest.cpp

To: mwolff, dfaure
Cc: #frameworks
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