selftests/ftrace: Add trace_pipe testcase

#!/bin/sh

# SPDX-License-Identifier: GPL-2.0

# description: trace_pipe and trace_marker

# flags: instance

[ ! -f trace_marker ] && exit_unsupported

echo "test input 1" > trace_marker

: "trace interface never consume the ring buffer"

grep -q "test input 1" trace

grep -q "test input 1" trace

: "trace interface never consume the ring buffer"

head -n 1 trace_pipe | grep -q "test input 1"

! grep -q "test input 1" trace