selftests: exec: revert to default emit rule

With the previous patch, the installation method change from install
to rsync. There is no need to create subdir during test, the
default EMIT_TESTS is enough.

This patch essentially revert commit 84cbd9e4 ("selftests/exec: do not
install subdir as it is already created").

Suggested-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>