test: attempt to install only kernel modules for each subsystem

test: attempt to install only kernel modules for each subsystem

When `linux-headers` is installed on Arch Linux, it stores the module
source tree in the kernel module directory, which is then picked up by
`find` and we get a lot of harmless but annoying errors:

```
...
modprobe: FATAL: Module Kconfig.iosched not found in directory /lib/modules/5.13.7-arch1-1
modprobe: FATAL: Module Kconfig not found in directory /lib/modules/5.13.7-arch1-1
modprobe: FATAL: Module Kconfig not found in directory /lib/modules/5.13.7-arch1-1
modprobe: FATAL: Module dm-mpath.h not found in directory /lib/modules/5.13.7-arch1-1
modprobe: FATAL: Module dm-bio-prison-v2.h not found in directory /lib/modules/5.13.7-arch1-1
modprobe: FATAL: Module raid0.h not found in directory /lib/modules/5.13.7-arch1-1
...
```

Let's fix this by trying to install only kernel modules (*.ko files with
an optional compression).