tar: avoid assumptions about root access and chmod -w in test cases

* tests/extrac07.at, tests/extrac09.at, tests/listed03.at: Use
AT_UNPRIVILEGED_PREREQ, since this test requires non-root
privileges.
* tests/extrac07.at: Don't use "chmod -w", as POSIX says it's not
portable to start a chmod permissions-list with "-" as it may be
confused with an option.  Use "chmod a-w" instead.

tests/extrac07.at

@@ -28,13 +28,15 @@ AT_SETUP([extracting symlinks to a read-only dir])
 AT_KEYWORDS([extract extract07 read-only symlink])
 
 AT_TAR_CHECK([
+AT_UNPRIVILEGED_PREREQ
+
 echo Prepare the directory
 mkdir dir
 genfile -f foo
 cd dir
 ln -s ../foo .
 cd ..
-chmod -w dir
+chmod a-w dir
 
 echo Create the archive
 tar cf archive dir || exit 1

tests/extrac09.at

@@ -23,6 +23,8 @@ AT_SETUP([no need to save dir with unreadable . and ..])
 AT_KEYWORDS([extract extrac09])
 
 AT_TAR_CHECK([
+AT_UNPRIVILEGED_PREREQ
+
 mkdir dir
 mkdir dir/sub
 mkdir dir/sub/extract

tests/listed03.at

@@ -23,6 +23,8 @@ AT_SETUP([incremental dump when the parent directory is unreadable])
 AT_KEYWORDS([listed incremental listed03])
 
 AT_TAR_CHECK([
+AT_UNPRIVILEGED_PREREQ
+
 mkdir dir
 mkdir dir/sub
 mkdir dir/sub/a