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/du/long-sloop b/tests/du/long-sloop
index 78926d38f..f9e70ad00 100755
--- a/tests/du/long-sloop
+++ b/tests/du/long-sloop
@@ -29,11 +29,11 @@ print_ver_ du
 # in a single directory.
 n=400
 
-dir_list=`seq $n`
+dir_list=$(seq $n)
 mkdir $dir_list || framework_failure_
 file=1
 i_minus_1=0
-for i in $dir_list `expr $n + 1`; do
+for i in $dir_list $(expr $n + 1); do
   case $i_minus_1 in
   0) ;;
   *)
@@ -56,7 +56,7 @@ echo foo > $i
 cat $file > /dev/null 2> err &&
     skip_ 'Your system appears to be able to handle more than $n symlinks
 in file name resolution'
-too_many=`sed 's/.*: //' err`
+too_many=$(sed 's/.*: //' err)
 
 
 # With coreutils-5.93 there was no failure.