tests: nproc: support referencing of external command

* tests/nproc/nproc-quota.sh: Use command -v nproc instead of
$abs_top_builddir for external usage of tests.
Also quote "$nproc" usage, to support spaces in path.

Link: https://github.com/coreutils/coreutils/pull/310

tests/nproc/nproc-quota.sh

@@ -44,10 +44,10 @@ ROOT=cgroup
   echo 'max 100000' > $ROOT/sys/fs/cgroup/foo/cpu.max  # ignored
 } || framework_failure_
 
-nproc=$abs_top_builddir/src/nproc$EXEEXT
-cp --parents $(ldd $nproc | grep -o '/[^ ]*') $ROOT ||
+nproc=$(command -v nproc)
+cp --parents $(ldd "$nproc" | grep -o '/[^ ]*') $ROOT ||
   skip_ 'Failed to copy nproc libs to chroot'
-cp $nproc $ROOT || framework_failure_
+cp "$nproc" $ROOT || framework_failure_
 
 NPROC() {
   IFS=': ' read key policy < $ROOT/proc/self/sched