kernel_module test breaks ninja build

cmake -GNinja results in:

```
CMake Warning (dev):
Policy CMP0058 is not set: Ninja requires custom command byproducts to be
explicit. Run "cmake --help-policy CMP0058" for policy details. Use the
cmake_policy command to set the policy and suppress this warning.

This project specifies custom command DEPENDS on files in the build tree
that are not specified as the OUTPUT or BYPRODUCTS of any
add_custom_command or add_custom_target:

tests/kernel_module/Kbuild

For compatibility with versions of CMake that did not have the BYPRODUCTS
option, CMake is generating phony rules for such files to convince 'ninja'
to build.

Project authors should add the missing BYPRODUCTS or OUTPUT options to the
custom commands that produce these files.
```