perf test: Add test case for checking number of EXIT events

The new test__task_exit() test runs a simple "/usr/bin/true" workload and then
checks whether the number of EXIT event is 1 or not.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/87obeljax4.fsf@sejong.aot.lge.com
[ committer note: Fixup conflicts with f4c66b4 ( bp overflow tests ) ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>