ftracetest: Fix hist unsupported result in hist selftests

ftracetest: Fix hist unsupported result in hist selftests

When histograms are not configured in the kernel, the ftracetest histogram
selftests should return "unsupported" and not "Failed". To detect this, the
test scripts have:

 FEATURE=`grep hist events/sched/sched_process_fork/trigger`
 if [ -z "$FEATURE" ]; then
     echo "hist trigger is not supported"
     exit_unsupported
 fi

The problem is that '-e' is in effect and any error will cause the program
to terminate. The grep for 'hist' fails, because it is not compiled it (thus
unsupported), but because grep has an error code for failing to find the
string, it causes the program to terminate, and is marked as a failed test.

Namhyung Kim recommended to test for the "hist" file located in
events/sched/sched_process_fork/hist instead, as it is more inline with the
other checks. As the hist file is only created if the histogram feature is
enabled, that is a valid check.

Link: http://lkml.kernel.org/r/20160523151538.4ea9ce0c@gandalf.local.home
Suggested-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: d6de7b888278a ("kselftests/ftrace: Add hist trigger testcases")
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>