tests: avoid spurious "make check-root" failure

* README (Running tests as root): Also set PATH in suggested "sudo"
command.  This avoids failure of at least tests/cp/cp-a-selinux
when the default PATH does not contain /sbin.
* tests/cp/cp-a-selinux: Don't redirect stderr to /dev/null.
"mkfs" was failing due to /sbin not being in PATH.

README

@@ -145,7 +145,7 @@ I find that it is best to unpack and build as a non-privileged
 user, and then to run the following command as that user in order
 to run the privilege-requiring tests:
 
-  sudo env NON_ROOT_USERNAME=$USER make -k check-root
+  sudo env PATH="$PATH" NON_ROOT_USERNAME=$USER make -k check-root
 
 If you can run the tests as root, please do so and report any
 problems.  We get much less test coverage in that mode, and it's

tests/cp/cp-a-selinux

@@ -34,7 +34,7 @@ cleanup_() { cd /; umount "$cwd/mnt"; }
 dd if=/dev/zero of=blob bs=8192 count=200 > /dev/null 2>&1 \
                                              || framework_failure
 mkdir mnt                                    || framework_failure
-mkfs -t ext2 -F blob > /dev/null 2>&1        || framework_failure
+mkfs -t ext2 -F blob                         || framework_failure
 
 # This context is special: it works even when mcstransd isn't running.
 ctx=root:object_r:tmp_t:s0