selftests: ftrace: Fix trigger extended error testcase

selftests: ftrace: Fix trigger extended error testcase

Previous testcase redirects echo-out into /dev/null
using "&>" as below

echo "trigger-command" >> trigger &> /dev/null

But this means redirecting both stdout and stderr into
/dev/null because it is same as below

echo "trigger-command" >> trigger > /dev/null 2>&1

So ">> trigger" redirects stdout to trigger file, but
next "> /dev/null" redirects stdout to /dev/null again
and the last "2>/&1" redirects stderr to stdout (/dev/null)

This fixes it by "2> /dev/null". And also, since it
must fail, add "!" to echo command.

Link: http://lkml.kernel.org/r/152292052250.15769.12565292689264162435.stgit@devbox
Fixes: f06eec4d0f2c ("selftests: ftrace: Add inter-event hist triggers testcases")
Reviewed-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>