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, 13 insertions, 13 deletions

diff --git a/tests/mkdir/p-1 b/tests/mkdir/p-1
index a2c41455d..cd8e67606 100755
--- a/tests/mkdir/p-1
+++ b/tests/mkdir/p-1
@@ -19,7 +19,7 @@
 . "${srcdir=.}/init.sh"; path_prepend_ ../src
 print_ver_ mkdir
 
-mkdir --parents "`pwd`/t" || fail=1
+mkdir --parents "$(pwd)/t" || fail=1
 test -d t || fail=1
 
 Exit $fail
diff --git a/tests/mkdir/p-2 b/tests/mkdir/p-2
index 7d1d09501..d0c487ec9 100755
--- a/tests/mkdir/p-2
+++ b/tests/mkdir/p-2
@@ -19,7 +19,7 @@
 . "${srcdir=.}/init.sh"; path_prepend_ ../src
 print_ver_ mkdir
 
-mkdir --parents "`pwd`/t/u" || fail=1
+mkdir --parents "$(pwd)/t/u" || fail=1
 test -d t/u || fail=1
 
 Exit $fail
diff --git a/tests/mkdir/p-3 b/tests/mkdir/p-3
index 90cbd9920..7031b2ede 100755
--- a/tests/mkdir/p-3
+++ b/tests/mkdir/p-3
@@ -26,7 +26,7 @@ mkdir no-access || framework_failure_
 mkdir no-acce2s || framework_failure_
 mkdir -p no-acce3s/d || framework_failure_
 
-p=`pwd`
+p=$(pwd)
 (cd no-access && chmod 0 . && mkdir -p "$p/a/b" u/v) 2> /dev/null && fail=1
 test -d "$p/a/b" || fail=1
 
@@ -44,7 +44,7 @@ if is_local_dir_ .; then
   test -d "$p/b/c" || fail=1
 fi
 
-b=`ls "$p/a" | tr -d '\n'`
+b=$(ls "$p/a" | tr -d '\n')
 # With coreutils-5.3.0, this would fail with $b=bu.
 test "x$b" = xb || fail=1
 
diff --git a/tests/mkdir/parents b/tests/mkdir/parents
index c9bf37bb0..0543e675e 100755
--- a/tests/mkdir/parents
+++ b/tests/mkdir/parents
@@ -31,21 +31,21 @@ mkdir e-dir > /dev/null 2>&1 && fail=1
 # Create an existing directory.
 umask 077
 mode_str=drwxr-x-wx
-mode_arg=`rwx_to_mode_ $mode_str`
+mode_arg=$(rwx_to_mode_ $mode_str)
 mkdir -m $mode_arg a || fail=1
 
 # this 'mkdir -p ...' shouldn't change perms of existing dir 'a'.
 d_mode_str=drwx-w--wx
-d_mode_arg=`rwx_to_mode_ $d_mode_str`
+d_mode_arg=$(rwx_to_mode_ $d_mode_str)
 mkdir -p -m $d_mode_arg a/b/c/d
 
 # Make sure the permissions of 'a' haven't been changed.
-p=`ls -ld a|cut -b-10`; case $p in $mode_str);; *) fail=1;; esac
+p=$(ls -ld a|cut -b-10); case $p in $mode_str);; *) fail=1;; esac
 # 'b's and 'c's should reflect the umask
-p=`ls -ld a/b|cut -b-10`; case $p in drwx------);; *) fail=1;; esac
-p=`ls -ld a/b/c|cut -b-10`; case $p in drwx------);; *) fail=1;; esac
+p=$(ls -ld a/b|cut -b-10); case $p in drwx------);; *) fail=1;; esac
+p=$(ls -ld a/b/c|cut -b-10); case $p in drwx------);; *) fail=1;; esac
 
 # 'd's perms are determined by the -m argument.
-p=`ls -ld a/b/c/d|cut -b-10`; case $p in $d_mode_str);; *) fail=1;; esac
+p=$(ls -ld a/b/c/d|cut -b-10); case $p in $d_mode_str);; *) fail=1;; esac
 
 Exit $fail
diff --git a/tests/mkdir/perm b/tests/mkdir/perm
index cabae6cc9..f9b207f81 100755
--- a/tests/mkdir/perm
+++ b/tests/mkdir/perm
@@ -38,7 +38,7 @@ tests='
     027  :   -m =+X   : drwxr-x--- : d--x--x--- :
     -    :   -        : last       : last       :
     '
-colon_tests=`echo $tests | sed 's/^ *//; s/ *: */:/g'`
+colon_tests=$(echo $tests | sed 's/^ *//; s/ *: */:/g')
 
 for p in empty -p; do
   test _$p = _empty && p=
diff --git a/tests/mkdir/special-1 b/tests/mkdir/special-1
index fbf3f1f7b..d478930eb 100755
--- a/tests/mkdir/special-1
+++ b/tests/mkdir/special-1
@@ -26,7 +26,7 @@ tmp=t
 mkdir -m$set_mode_string $tmp || fail=1
 
 test -d $tmp || fail=1
-mode=`ls -ld $tmp|cut -b-10`
+mode=$(ls -ld $tmp|cut -b-10)
 case "$mode" in
   $output_mode_string) ;;
   *) fail=1 ;;
@@ -42,7 +42,7 @@ mkdir -m$set_mode_string $tmp2 2> /dev/null && fail=1
 mkdir --parents -m$set_mode_string $tmp2 || fail=1
 
 test -d $tmp2 || fail=1
-mode=`ls -ld $tmp2|cut -b-10`
+mode=$(ls -ld $tmp2|cut -b-10)
 case "$mode" in
   $output_mode_string) ;;
   *) fail=1 ;;