tests: change `...' to '...' on lines not matching /[=\$]/

Exempt lines with '$' or '=', since those are prone to improper
conversion.  Run this:
  git grep -l "\`[^']*'" tests \
   |xargs perl -pi -e '/[=\$]/ and next;s/\`([^'\''"]*?'\'')/'\''$1/g'

tests/mkdir/p-v

@@ -22,11 +22,11 @@ print_ver_ mkdir
 mkdir -pv foo/a/b/c/d >out || fail=1
 
 compare - out <<\EOF
-mkdir: created directory `foo'
-mkdir: created directory `foo/a'
-mkdir: created directory `foo/a/b'
-mkdir: created directory `foo/a/b/c'
-mkdir: created directory `foo/a/b/c/d'
+mkdir: created directory 'foo'
+mkdir: created directory 'foo/a'
+mkdir: created directory 'foo/a/b'
+mkdir: created directory 'foo/a/b/c'
+mkdir: created directory 'foo/a/b/c/d'
 EOF
 
 Exit $fail

tests/mkdir/parents

@@ -23,8 +23,8 @@ skip_if_setgid_
 mkdir -m 700 e-dir || framework_failure_
 
 
-# Make sure `mkdir -p existing-dir' succeeds
-# and that `mkdir existing-dir' fails.
+# Make sure 'mkdir -p existing-dir' succeeds
+# and that 'mkdir existing-dir' fails.
 mkdir -p e-dir || fail=1
 mkdir e-dir > /dev/null 2>&1 && fail=1
 
@@ -34,18 +34,18 @@ mode_str=drwxr-x-wx
 mode_arg=`rwx_to_mode_ $mode_str`
 mkdir -m $mode_arg a || fail=1
 
-# this `mkdir -p ...' shouldn't change perms of existing dir `a'.
+# this 'mkdir -p ...' shouldn't change perms of existing dir 'a'.
 d_mode_str=drwx-w--wx
 d_mode_arg=`rwx_to_mode_ $d_mode_str`
 mkdir -p -m $d_mode_arg a/b/c/d
 
-# Make sure the permissions of `a' haven't been changed.
+# Make sure the permissions of 'a' haven't been changed.
 p=`ls -ld a|cut -b-10`; case $p in $mode_str);; *) fail=1;; esac
-# `b's and `c's should reflect the umask
+# 'b's and 'c's should reflect the umask
 p=`ls -ld a/b|cut -b-10`; case $p in drwx------);; *) fail=1;; esac
 p=`ls -ld a/b/c|cut -b-10`; case $p in drwx------);; *) fail=1;; esac
 
-# `d's perms are determined by the -m argument.
+# 'd's perms are determined by the -m argument.
 p=`ls -ld a/b/c/d|cut -b-10`; case $p in $d_mode_str);; *) fail=1;; esac
 
 Exit $fail

tests/mkdir/perm

@@ -1,5 +1,5 @@
 #!/bin/sh
-# Verify that mkdir's `-m MODE' option works properly
+# Verify that mkdir's '-m MODE' option works properly
 # with various umask settings.
 
 # Copyright (C) 2000, 2002-2012 Free Software Foundation, Inc.