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

1 files changed, 2 insertions, 2 deletions

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 ;;