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, 3 insertions, 3 deletions

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
   '')