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

6 files changed, 7 insertions, 7 deletions

diff --git a/tests/rm/hash b/tests/rm/hash
index dc443e1fd..344325e13 100755
--- a/tests/rm/hash
+++ b/tests/rm/hash
@@ -24,7 +24,7 @@ expensive_
 
 # Create a hierarchy with 3*26 leaf directories, each at depth 153.
 echo "$0: creating 78 trees, each of depth 153; this will take a while..." >&2
-y=`seq 1 150|tr -sc '\n' y|tr '\n' /`
+y=$(seq 1 150|tr -sc '\n' y|tr '\n' /)
 for i in 1 2 3; do
   for j in a b c d e f g h i j k l m n o p q r s t u v w x y z; do
     mkdir -p t/$i/$j/$y || framework_failure_
diff --git a/tests/rm/inaccessible b/tests/rm/inaccessible
index e9e1788ea..2f045815e 100755
--- a/tests/rm/inaccessible
+++ b/tests/rm/inaccessible
@@ -25,7 +25,7 @@ print_ver_ rm
 require_openat_support_
 skip_if_root_
 
-p=`pwd`
+p=$(pwd)
 mkdir abs1 abs2 no-access || framework_failure_
 
 
diff --git a/tests/rm/ir-1 b/tests/rm/ir-1
index ac2959020..01b33bc1e 100755
--- a/tests/rm/ir-1
+++ b/tests/rm/ir-1
@@ -48,7 +48,7 @@ rm --verbose -i -r $t < in > /dev/null 2>&1 || fail=1
 test -d $t || fail=1
 
 # There should be only one directory left.
-case `echo $t/*` in
+case $(echo $t/*) in
   $t/[abc]) ;;
   *) fail=1 ;;
 esac
diff --git a/tests/rm/read-only b/tests/rm/read-only
index e530dcee3..018648be0 100755
--- a/tests/rm/read-only
+++ b/tests/rm/read-only
@@ -20,7 +20,7 @@
 print_ver_ rm
 require_root_
 
-cwd=`pwd`
+cwd=$(pwd)
 cleanup_() { cd /; umount "$cwd/mnt"; }
 
 skip=0
diff --git a/tests/rm/readdir-bug b/tests/rm/readdir-bug
index 939ce9862..711e1bc0e 100755
--- a/tests/rm/readdir-bug
+++ b/tests/rm/readdir-bug
@@ -26,8 +26,8 @@ print_ver_ rm
 # http://lists.gnu.org/archive/html/bug-coreutils/2006-09/msg00326.html
 mkdir b || framework_failure_
 cd b || framework_failure_
-for i in `seq 1 250`; do
-  touch `printf %040d $i` || framework_failure_
+for i in $(seq 1 250); do
+  touch $(printf %040d $i) || framework_failure_
 done
 cd .. || framework_failure_
 
diff --git a/tests/rm/unread3 b/tests/rm/unread3
index 336c05a1b..4b8bbc2d3 100755
--- a/tests/rm/unread3
+++ b/tests/rm/unread3
@@ -23,7 +23,7 @@ skip_if_root_
 mkdir -p a/1 b c d/2 e/3 || framework_failure_
 
 
-t=`pwd`
+t=$(pwd)
 cd c
 chmod u=x,go= .