selftests: Move kselftest_module.sh into kselftest/

The kselftest_module.sh file was not being installed by the Makefile
"install" target, rendering the lib/*.sh tests nonfunction. This fixes
that and takes the opportunity to move it into the kselftest/ subdirectory
which is where the kselftest infrastructure bits are collecting.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Link: https://lore.kernel.org/lkml/CA+G9fYsfJpXQvOvHdjtg8z4a89dSStOQZOKa9zMjjQgWKng1aw@mail.gmail.com
Fixes: d3460527

 ("kselftest: Add test runner creation script")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>