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, 6 insertions, 6 deletions

diff --git a/tests/mv/part-symlink b/tests/mv/part-symlink
index 30cb4400e..2e7cf1b8b 100755
--- a/tests/mv/part-symlink
+++ b/tests/mv/part-symlink
@@ -27,7 +27,7 @@ cleanup_() { rm -rf "$other_partition_tmpdir"; }
 # mv: preserving permissions for 'rem_sl': Operation not supported
 require_local_dir_
 
-pwd_tmp=`pwd`
+pwd_tmp=$(pwd)
 
 # Unset CDPATH.  Otherwise, output from the 'cd dir' command
 # can make this test fail.
@@ -68,15 +68,15 @@ for copy in cp mv; do
       rm -f "$other_partition_tmpdir"/* || fail=1
       mkdir dir || fail=1
       cd dir || fail=1
-      case "$args" in *loc_reg*) reg_abs="`pwd`/$loc_reg" ;; esac
+      case "$args" in *loc_reg*) reg_abs="$(pwd)/$loc_reg" ;; esac
       case "$args" in *rem_reg*) reg_abs=$rem_reg ;; esac
       case "$args" in *loc_sl*) slink=$loc_sl ;; esac
       case "$args" in *rem_sl*) slink=$rem_sl ;; esac
 
       echo $contents > "$reg_abs" || fail=1
       ln -nsf "$reg_abs" $slink || fail=1
-      actual_args=`echo $args|sed 's,^,$,;s/ / $/'`
-      actual_args=`eval echo $actual_args`
+      actual_args=$(echo $args|sed 's,^,$,;s/ / $/')
+      actual_args=$(eval echo $actual_args)
 
       (
         (
@@ -115,7 +115,7 @@ for copy in cp mv; do
             for f in $actual_args; do
               test -f $f ||
                 { echo " $copy FAILED but removed $f"; continue; }
-              case "`cat $f`" in
+              case "$(cat $f)" in
                 "$contents") ;;
                 *) echo " $copy FAILED but modified $f";;
               esac
@@ -130,7 +130,7 @@ for copy in cp mv; do
                 if test $copy_status != 0; then
                   test
                 fi
-                case "`cat $f`" in
+                case "$(cat $f)" in
                   "$contents") ;;
                   *) echo " $copy FAILED";;
                 esac