selftests/ftrace: Add checkbashisms meta-testcase

Add a meta-testcase which tests ftracetest itself with
checkbasisms. This helps us to keep our test script
bashisms clean.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>