selftests: ftrace: Check whether snapshot trigger is supported correctly

If "snapshot" special file doesn't exist, that kernel does
not support snapshot and snapshot trigger too. In that case
snapshot trigger test results to unsupported instead of fail.

Link: http://lkml.kernel.org/r/147928401215.22982.10411665829041109794.stgit@devbox

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

tools/testing/selftests/ftrace/test.d/trigger/trigger-snapshot.tc

@@ -23,6 +23,11 @@ if [ ! -f events/sched/sched_process_fork/trigger ]; then
     exit_unsupported
 fi
 
+if [ ! -f snapshot ]; then
+    echo "snapshot is not supported"
+    exit_unsupported
+fi
+
 reset_tracer
 do_reset