selftests/ftrace: Return unsupported if no error_log file

Check whether error_log file exists in tracing/error_log testcase
and return UNSUPPORTED if no error_log file.

This can happen if we run the ftracetest on the older stable
kernel.

Fixes: 4eab1cc4

 ("selftests/ftrace: Add tracing/error_log testcase")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>