selftests: remove useless TEST_DIRS

The TEST_DIRS was introduced in Commit e8c1d7cd ("selftests: copy
TEST_DIRS to INSTALL_PATH") for coping a whole directory in ftrace.

After rsync(with -a) is introduced by Commit 900d65ee ("selftests:
change install command to rsync"). Rsync could handle the directory
without the definition of TEST_DIRS.

This patch simply replace TEST_DIRS with TEST_FILES in ftrace and remove
the TEST_DIRS in tools/testing/selftest/lib.mk

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>