install: cannot stat 'testsuite/module-playground/mod-loop-f.ko': No
such file or directory
Makefile:2881: recipe for target 'rootfs' failed
make[1]: *** [rootfs] Error 1
make[1]: *** Waiting for unfinished jobs....
Makefile:2101: recipe for target 'check-recursive' failed
We need to ship pre-compiled binaries so it's possible to run
"make check" on servers without kernel headers.
Also add them to EXTRA_DIST as other sources.