tests: adjust \`...' quoting to '...' to adapt to latest gnulib

Run this:
  git grep -l "\\\\\`[^']*'" tests \
    |xargs perl -pi -e 's/\\\`(.*?'\'')/'\''$1/g

tests/chmod/c-option

@@ -33,7 +33,7 @@ chmod -c g=rwx $file > empty || fail=1
 
 test -s empty && fail=1
 case "`cat out`" in
-  "mode of \`f' changed from 0744 "?rwxr--r--?" to 0774 "?rwxrwxr--?) ;;
+  "mode of 'f' changed from 0744 "?rwxr--r--?" to 0774 "?rwxrwxr--?) ;;
   *) cat out; fail=1 ;;
 esac
 

tests/chmod/equal-x

@@ -27,7 +27,7 @@ for mode in =x =xX =Xx =x,=X =X,=x; do
   chmod a=r,$mode $file || fail=1
   case "`ls -l $file`" in
     ---x--x---*) ;;
-    *) fail=1; echo "after \`chmod $mode $file':"; ls -l $file ;;
+    *) fail=1; echo "after 'chmod $mode $file':"; ls -l $file ;;
   esac
 done
 

tests/chmod/no-x

@@ -41,7 +41,7 @@ sed "s/^$prog: cannot read directory /$prog: /" out > t && mv t out
 sed 's,d/no-x/y,d/no-x,' out > t && mv t out
 
 cat <<EOF > exp
-$prog: \`d/no-x': Permission denied
+$prog: 'd/no-x': Permission denied
 EOF
 
 compare exp out || fail=1

tests/chmod/thru-dangling

@@ -25,7 +25,7 @@ ln -s non-existent dangle || framework_failure_
 # This operation cannot succeed since the symbolic link dangles.
 chmod 644 dangle 2> out && fail=1
 
-echo "chmod: cannot operate on dangling symlink \`dangle'" > exp
+echo "chmod: cannot operate on dangling symlink 'dangle'" > exp
 compare exp out || fail=1
 
 Exit $fail