perf tools: Allow to reset open files counter

perf tools: Allow to reset open files counter

I hit a bug when running test suite without forking
each test (-F option):

  $ perf test -F dso
   8: Test dso data read                                       : Ok
   9: Test dso data cache                                      : FAILED!
  10: Test dso data reopen                                     : FAILED!

The reason the session file limit is set just once for
perf process so we need to reset it for each test,
otherwise wrong limit is taken into account.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Nilay Vaish <nilayvaish@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1467113345-12669-2-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>