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/chgrp/posix-H b/tests/chgrp/posix-H
index a6474f562..3965f1ff5 100755
--- a/tests/chgrp/posix-H
+++ b/tests/chgrp/posix-H
@@ -46,7 +46,7 @@ for i in $changed; do
   # Filter out symlinks (entries that end in 's'), since it's not
   # possible to change their group/owner information on some systems.
   case $i in *s) continue;; esac
-  set _ `ls -dgn $i`; shift
+  set _ $(ls -dgn $i); shift
   group=$3
   test $group = $g2 || fail=1
 done
@@ -62,7 +62,7 @@ for i in $not_changed; do
   # Filter out symlinks (entries that end in 's'), since it's not
   # possible to change their group/owner information on some systems.
   case $i in *s) continue;; esac
-  set _ `ls -dgn $i`; shift
+  set _ $(ls -dgn $i); shift
   group=$3
   test $group = $g1 || fail=1
 done