tests: avoid test framework failure if the file system lacks ACL support

Prompted by a test framework failure of tests/mkdir/p-acl.sh on VMs
where filesystem are mounted without ACL support.
I pushed the following patch upstream which will go into 8.23 some day.
_______________________________________________

http://git.sv.gnu.org/cgit/coreutils.git/commit/?id=5d7591d0

commit 5d7591d0edf0dd31c2daa195ee766c1383b89f4c
Author: Bernhard Voelker <mail@bernhard-voelker.de>
Date:   Fri Jan 10 16:48:25 2014 +0100

    tests: improve test for a working setfacl
    
    Prompted by a test framework failure of tests/mkdir/p-acl.sh on armv7l:
    The previous test for a working setfacl was not sufficient in some
    circumstances.
    
    * init.cfg (require_setfacl_): Call setfacl twice with conflictive
    ACL specs, and use ACL specs which can't be mapped into regular file
    permission bits.  Document the reasons.
---
 init.cfg |   32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

Index: init.cfg
===================================================================
--- init.cfg.orig	2014-02-24 15:19:12.185467061 +0100
+++ init.cfg	2014-02-24 15:20:46.802273660 +0100
@@ -192,10 +192,38 @@ require_valgrind_()
     skip_ "requires a working valgrind"
 }
 
+# Skip the current test if setfacl doesn't work on the current file system,
+# which could happen if not installed, or if ACLs are not supported by the
+# kernel or the file system, or are turned off via mount options.
+#
+# Work around the following two issues:
+#
+# 1) setfacl maps ACLs into file permission bits if on "noacl" file systems.
+#
+# On file systems which do not support ACLs (e.g. ext4 mounted with -o noacl),
+# setfacl operates on the regular file permission bits, and only fails if the
+# given ACL spec does not fit into there.  Thus, to test if ACLs really work
+# on the current file system, pass an ACL spec which can't be mapped that way.
+# "Default" ACLs (-d) seem to fulfill this requirement.
+#
+# 2) setfacl only invokes the underlying system call if the ACL would change.
+#
+# If the given ACL spec would not change the ACLs on the file, then setfacl
+# does not invoke the underlying system call - setxattr().  Therefore, to test
+# if setting ACLs really works on the current file system, call setfacl twice
+# with conflictive ACL specs.
 require_setfacl_()
 {
-  setfacl -m user::rwx . \
-    || skip_ "setfacl does not work on the current file system"
+  local d='acltestdir_'
+  mkdir $d || framework_failure_
+  local f=0
+
+  setfacl -d -m user::r-x $d \
+    && setfacl -d -m user::rwx $d \
+    || f=1
+  rm -rf $d || framework_failure_
+  test $f = 0 \
+     || skip_ "The current file system does not support ACLs"
 }
 
 # Require a controlling input 'terminal'.