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, 1 insertions, 1 deletions

diff --git a/tests/CuTmpdir.pm b/tests/CuTmpdir.pm
index 05c302995..efa646213 100644
--- a/tests/CuTmpdir.pm
+++ b/tests/CuTmpdir.pm
@@ -28,7 +28,7 @@ my $dir;
 
 sub skip_test($)
 {
-  warn "$ME: skipping test: unsafe working directory name: `$_[0]'\n";
+  warn "$ME: skipping test: unsafe working directory name: '$_[0]'\n";
   exit 77;
 }