tests: more automated quote adjustment

Relax initial regexp to match more instances, but add a
filter to avoid some invalid conversions.  Run this:

git grep -l "\`[^']*'" tests | xargs perl -pi -e '$q=q"'\''";' \
  -e '$q="$q\\$q$q"; /(= ?\`|\`expr|\`echo|\Q$q\E)/ and next;' \
  -e ' s/\`([^'\''"]*?'\'')/'\''$1/g'
The last disjunct in the above (...) filter is to exempt
any line that contains this string: '\''
With quoting like that, converting a ` to ' is likely to cause trouble,
so we'll handle those manually.  Here are three examples where
the exemption is required:

  *': `link-to-dir/'\'': hard link not allowed for directory'*) ;;
  printf 'creating file `%s'\''\n' $f
  'mv: inter-device move failed: `%s'\'' to `%s'\'';'\

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/du/files0-from b/tests/du/files0-from
index c62c2a805..67e1e2e76 100755
--- a/tests/du/files0-from
+++ b/tests/du/files0-from
@@ -30,14 +30,14 @@ my @Tests =
   (
    # invalid extra command line argument
    ['f-extra-arg', '--files0-from=- no-such', {IN=>"a"}, {EXIT=>1},
-    {ERR => "$prog: extra operand `no-such'\n"
+    {ERR => "$prog: extra operand 'no-such'\n"
         . "file operands cannot be combined with --files0-from\n"
         . "Try '$prog --help' for more information.\n"}
     ],
 
    # missing input file
    ['missing', '--files0-from=missing', {EXIT=>1},
-    {ERR => "$prog: cannot open `missing' for reading: "
+    {ERR => "$prog: cannot open 'missing' for reading: "
      . "No such file or directory\n"}],
 
    # input file name of '-'