selftests/bpf: Test function_graph tracer and bpf trampoline together

Add simple test script to execute funciton graph tracer while BPF trampoline
attaches and detaches from the functions being graph traced.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191209000114.1876138-4-ast@kernel.org

tools/testing/selftests/bpf/test_ftrace.sh

+#!/bin/bash
+
+TR=/sys/kernel/debug/tracing/
+clear_trace() { # reset trace output
+    echo > $TR/trace
+}
+
+disable_tracing() { # stop trace recording
+    echo 0 > $TR/tracing_on
+}
+
+enable_tracing() { # start trace recording
+    echo 1 > $TR/tracing_on
+}
+
+reset_tracer() { # reset the current tracer
+    echo nop > $TR/current_tracer
+}
+
+disable_tracing
+clear_trace
+
+echo "" > $TR/set_ftrace_filter
+echo '*printk* *console* *wake* *serial* *lock*' > $TR/set_ftrace_notrace
+
+echo "bpf_prog_test*" > $TR/set_graph_function
+echo "" > $TR/set_graph_notrace
+
+echo function_graph > $TR/current_tracer
+
+enable_tracing
+./test_progs -t fentry
+./test_progs -t fexit
+disable_tracing
+clear_trace
+
+reset_tracer
+
+exit 0