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, 4 insertions, 4 deletions

diff --git a/tests/chown/separator b/tests/chown/separator
index 6295fd78f..d4264a8c8 100755
--- a/tests/chown/separator
+++ b/tests/chown/separator
@@ -19,16 +19,16 @@
 . "${srcdir=.}/init.sh"; path_prepend_ ../src
 print_ver_ chown
 
-id_u=`id -u` || framework_failure_
+id_u=$(id -u) || framework_failure_
 test -n "$id_u" || framework_failure_
 
-id_un=`id -un` || framework_failure_
+id_un=$(id -un) || framework_failure_
 test -n "$id_un" || framework_failure_
 
-id_g=`id -g` || framework_failure_
+id_g=$(id -g) || framework_failure_
 test -n "$id_g" || framework_failure_
 
-id_gn=`id -gn` || framework_failure_
+id_gn=$(id -gn) || framework_failure_
 test -n "$id_gn" || framework_failure_
 
 # FreeBSD 6.x's getgrnam fails to look up a group name containing