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/cp/perm b/tests/cp/perm
index 2e6126d62..eb3925da2 100755
--- a/tests/cp/perm
+++ b/tests/cp/perm
@@ -49,8 +49,8 @@ for u in 31 37 2; do
 
             case "$cmd:$force:$existing_dest" in
               cp:*:yes)
-                _g_perm=`echo rwx|sed 's/[^'$g_perm']/-/g'`
-                _o_perm=`echo rwx|sed 's/[^'$o_perm']/-/g'`
+                _g_perm=$(echo rwx|sed 's/[^'$g_perm']/-/g')
+                _o_perm=$(echo rwx|sed 's/[^'$o_perm']/-/g')
                 expected_perms=-rw-$_g_perm$_o_perm
                 ;;
               cp:*:no)