selftests: warn if failure is due to lack of executable bit

Executing selftests is fragile as if someone forgot to set a secript
as executable the test will fail, and you won't know for sure if the
failure was caused by the lack of proper permissions or something else.

Setting scripts as executable is required, this also enable folks to
execute selftests as independent units.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>