tests: avoid realpath in test-lib.sh
tests: avoid realpath in test-lib.sh

Ever since commit ace4aecd, running 'make check' on RHEL 6 produces:

./test-lib.sh: line 21: realpath: command not found

for every shell script test, because 'realpath' was not part of
coreutils back then.

* tests/test-lib.sh (_scriptdir): Compute with only portable shell.

Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

andreabolognani/libvirt,nertpinx/libvirt,nertpinx/libvirt,eskultety/libvirt,libvirt/libvirt,nertpinx/libvirt,libvirt/libvirt,taget/libvirt,olafhering/libvirt,datto/libvirt,zippy2/libvirt,fabianfreyer/libvirt,rlaager/libvirt,jardasgit/libvirt,eskultety/libvirt,eskultety/libvirt,datto/libvirt,fabianfreyer/libvirt,datto/libvirt,VenkatDatta/libvirt,crobinso/libvirt,rlaager/libvirt,olafhering/libvirt,crobinso/libvirt,olafhering/libvirt,eskultety/libvirt,zippy2/libvirt,nertpinx/libvirt,jardasgit/libvirt,rlaager/libvirt,taget/libvirt,VenkatDatta/libvirt,zippy2/libvirt,fabianfreyer/libvirt,datto/libvirt,fabianfreyer/libvirt,crobinso/libvirt,VenkatDatta/libvirt,rlaager/libvirt,libvirt/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jfehlig/libvirt,taget/libvirt,jardasgit/libvirt,andreabolognani/libvirt,rlaager/libvirt,VenkatDatta/libvirt,olafhering/libvirt,jardasgit/libvirt,eskultety/libvirt,fabianfreyer/libvirt,taget/libvirt,taget/libvirt,datto/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jardasgit/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,libvirt/libvirt,crobinso/libvirt,andreabolognani/libvirt,zippy2/libvirt