selftests: ftrace: Introduce TMPDIR for temporary files

Introduce TMPDIR variable which is removed after each test
is done, so that the test script can put their temporary
files in that.

Link: http://lkml.kernel.org/r/147928406116.22982.8761924340108532378.stgit@devbox



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>