Makefile: export KERNELDIR for tests

We include linux/memfd.h now, make sure it's searched for in $KERNELDIR.
Makefile: deal with realtive paths in test/

Invoke realpath to convert a possible relative path into an absolute one.
test/Makefile: only look at $(KERNELDIR)/usr/include for files with quotes

This is a hack to make cpp look at system include files first, and then
consult $(KERNELDIR)/usr/include. For this to work, include directives
referring to files in uapi have to be addressed with quotes.