selftests/ftrace: Add a basic testcase for snapshot

This testcase is constrived to reproduce a problem that the cpu buffers
become unavailable which is due to 'record_disabled' of array_buffer and
max_buffer being messed up.

Local test result after bugfix:
  # ./ftracetest test.d/00basic/snapshot1.tc
  === Ftrace unit tests ===
  [1] Snapshot and tracing_cpumask        [PASS]
  [2] (instance)  Snapshot and tracing_cpumask    [PASS]

  # of passed:  2
  # of failed:  0
  # of unresolved:  0
  # of untested:  0
  # of unsupported:  0
  # of xfailed:  0
  # of undefined(test bug):  0

Link: https://lkml.kernel.org/r/20230805033816.3284594-3-zhengyejian1@huawei.com

Cc: <mhiramat@kernel.org>
Cc: <vnagarnaik@google.com>
Cc: <shuah@kernel.org>
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

tools/testing/selftests/ftrace/test.d/00basic/snapshot1.tc

+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: Snapshot and tracing_cpumask
+# requires: trace_marker tracing_cpumask snapshot
+# flags: instance
+
+# This testcase is constrived to reproduce a problem that the cpu buffers
+# become unavailable which is due to 'record_disabled' of array_buffer and
+# max_buffer being messed up.
+
+# Store origin cpumask
+ORIG_CPUMASK=`cat tracing_cpumask`
+
+# Stop tracing all cpu
+echo 0 > tracing_cpumask
+
+# Take a snapshot of the main buffer
+echo 1 > snapshot
+
+# Restore origin cpumask, note that there should be some cpus being traced
+echo ${ORIG_CPUMASK} > tracing_cpumask
+
+# Set tracing on
+echo 1 > tracing_on
+
+# Write a log into buffer
+echo "test input 1" > trace_marker
+
+# Ensure the log writed so that cpu buffers are still available
+grep -q "test input 1" trace
+exit 0