tests: convert nearly all `...` expressions to $(...)

Exempt init.sh because it runs before we're assured to have a
shell that groks $(...).  Exempt *.mk because "$" would have to
be doubled, and besides, any `...` expression in a .mk file is
almost certainly evaluated before init.sh is run.  Finally, also
exempt the perl-based tests, because perl's `...` cannot be
converted to $(...).  Do that by running this command:

git grep -l '`.*`' tests \
  | grep -Ev 'init\.sh|\.mk$' | xargs grep -Lw perl \
  | xargs perl -pi -e 's/`(.*?)`/\$($1)/g'

One minor fix-up change was required after that, due to how
quoting differs:
diff --git a/tests/chmod/equals b/tests/chmod/equals
-    expected_perms=$(eval 'echo \$expected_'$dest)
+    expected_perms=$(eval 'echo $expected_'$dest)

Another was to make these required quoting adjustments:
diff --git a/tests/misc/stty b/tests/misc/stty
...
-  rev=$(eval echo "\\\$REV_$opt")
+  rev=$(eval echo "\$REV_$opt")
...
-      rev1=$(eval echo "\\\$REV_$opt1")
-      rev2=$(eval echo "\\\$REV_$opt2")
+      rev1=$(eval echo "\$REV_$opt1")
+      rev2=$(eval echo "\$REV_$opt2")

Also, transform two files that were needlessly excluded above:
(both use perl, but are mostly bourne shell)

  perl -pi -e 's/`(.*?)`/\$($1)/g' \
    tests/du/long-from-unreadable tests/init.cfg

5 files changed, 10 insertions, 10 deletions

diff --git a/tests/chmod/c-option b/tests/chmod/c-option
index 9235b52fc..0fc83912a 100755
--- a/tests/chmod/c-option
+++ b/tests/chmod/c-option
@@ -32,7 +32,7 @@ chmod -c g=rwx $file > out || fail=1
 chmod -c g=rwx $file > empty || fail=1
 
 test -s empty && fail=1
-case "`cat out`" in
+case "$(cat out)" in
   "mode of 'f' changed from 0744 "?rwxr--r--?" to 0774 "?rwxrwxr--?) ;;
   *) cat out; fail=1 ;;
 esac
diff --git a/tests/chmod/equal-x b/tests/chmod/equal-x
index d2998d137..e2cac3478 100755
--- a/tests/chmod/equal-x
+++ b/tests/chmod/equal-x
@@ -25,7 +25,7 @@ touch $file || framework_failure_
 umask 005
 for mode in =x =xX =Xx =x,=X =X,=x; do
   chmod a=r,$mode $file || fail=1
-  case "`ls -l $file`" in
+  case "$(ls -l $file)" in
     ---x--x---*) ;;
     *) fail=1; echo "after 'chmod $mode $file':"; ls -l $file ;;
   esac
diff --git a/tests/chmod/equals b/tests/chmod/equals
index 54c39aedf..fd92055c2 100755
--- a/tests/chmod/equals
+++ b/tests/chmod/equals
@@ -32,15 +32,15 @@ for src in u g o; do
   for dest in u g o; do
     test $dest = $src && continue
     chmod a=,$src=rwx,$dest=$src,$src= f || fail=1
-    actual_perms=`ls -l f|cut -b-10`
-    expected_perms=`eval 'echo \$expected_'$dest`
+    actual_perms=$(ls -l f|cut -b-10)
+    expected_perms=$(eval 'echo $expected_'$dest)
     test "$actual_perms" = "$expected_perms" || fail=1
   done
 done
 
 umask 027
 chmod a=,u=rwx,=u f || fail=1
-actual_perms=`ls -l f|cut -b-10`
+actual_perms=$(ls -l f|cut -b-10)
 test "$actual_perms" = "-rwxr-x---" || fail=1
 
 Exit $fail
diff --git a/tests/chmod/setgid b/tests/chmod/setgid
index 081c36e0d..7fc6db214 100755
--- a/tests/chmod/setgid
+++ b/tests/chmod/setgid
@@ -31,7 +31,7 @@ chmod g+s d 2> /dev/null && env -- test -g d ||
     # to which you belong.  When that happens, the above chmod fails.  So
     # here, upon failure, we try to set the group, then rerun the chmod command.
 
-    id_g=`id -g` &&
+    id_g=$(id -g) &&
     test -n "$id_g" &&
     chgrp "$id_g" d &&
     chmod g+s d || framework_failure_
@@ -52,7 +52,7 @@ do
        expected_mode=drwxr-xr-x ;;
     *) expected_mode=drwxr-sr-x ;;
   esac
-  ls_output=`ls -ld d`
+  ls_output=$(ls -ld d)
   case $ls_output in
     $expected_mode*) ;;
     *) fail=1 ;;
diff --git a/tests/chmod/usage b/tests/chmod/usage
index 3a4d1b82a..9dd65f7cc 100755
--- a/tests/chmod/usage
+++ b/tests/chmod/usage
@@ -54,15 +54,15 @@ cases='
   ug,+x f    :
 '
 
-all_files=`echo "$cases" | sed 's/.*://'|sort -u`
+all_files=$(echo "$cases" | sed 's/.*://'|sort -u)
 
 old_IFS=$IFS
 IFS='
 '
 for case in $cases; do
   IFS=$old_IFS
-  args=`expr "$case" : ' *\(.*[^ ]\) *:'`
-  files=`expr "$case" : '.*: *\(.*\)'`
+  args=$(expr "$case" : ' *\(.*[^ ]\) *:')
+  files=$(expr "$case" : '.*: *\(.*\)')
 
   case $files in
   '')