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

4 files changed, 15 insertions, 15 deletions

diff --git a/tests/chown/basic b/tests/chown/basic
index 98145dac1..7c8951c83 100755
--- a/tests/chown/basic
+++ b/tests/chown/basic
@@ -25,12 +25,12 @@ touch f || framework_failure_
 chown -R --preserve-root 0:1 f
 
 # Make sure the owner and group are 0 and 1 respectively.
-set _ `ls -n f`; shift; test "$3:$4" = 0:1 || fail=1
+set _ $(ls -n f); shift; test "$3:$4" = 0:1 || fail=1
 
 # Make sure the correct diagnostic is output
 # Note we output a name even though an id was specified.
 chown -v --from=42 43 f > out || fail=1
-printf "ownership of 'f' retained as `id -nu`\n" > exp
+printf "ownership of 'f' retained as $(id -nu)\n" > exp
 compare exp out || fail=1
 
 # Ensure diagnostics work for non existent files.
@@ -41,19 +41,19 @@ compare exp out || fail=1
 chown --from=0:1 2:010 f || fail=1
 
 # And now they should be 2 and 10 respectively.
-set _ `ls -n f`; shift; test "$3:$4" = 2:10 || fail=1
+set _ $(ls -n f); shift; test "$3:$4" = 2:10 || fail=1
 
 ln -s f slink
 # Applying chown to a symlink with --no-dereference
 # should change only the link.
 chown --no-dereference 0:1 slink || fail=1
 # owner/group on the symlink should be set
-set _ `ls -n slink`; shift; test "$3:$4" = 0:1 || fail=1
+set _ $(ls -n slink); shift; test "$3:$4" = 0:1 || fail=1
 # owner/group on the referent should remain unchanged
-set _ `ls -n f`;     shift; test "$3:$4" = 2:10 || fail=1
+set _ $(ls -n f);     shift; test "$3:$4" = 2:10 || fail=1
 
 chown --no-dereference --from=0:1 2:010 slink || fail=1
 # owner/group on the symlink should be changed
-set _ `ls -n slink`; shift; test "$3:$4" = 2:10 || fail=1
+set _ $(ls -n slink); shift; test "$3:$4" = 2:10 || fail=1
 
 Exit $fail
diff --git a/tests/chown/deref b/tests/chown/deref
index 9ba28062e..d42a05412 100755
--- a/tests/chown/deref
+++ b/tests/chown/deref
@@ -23,7 +23,7 @@ print_ver_ chown
 ln -s no-such dangle || framework_failure_
 
 
-set _ `ls -ldo dangle`; shift; user=$3
+set _ $(ls -ldo dangle); shift; user=$3
 
 # With 5.2.1 and earlier, this command would mistakenly succeed.
 chown --dereference $user dangle 2> out1 && fail=1
diff --git a/tests/chown/preserve-root b/tests/chown/preserve-root
index a9145b7ab..ffad22228 100755
--- a/tests/chown/preserve-root
+++ b/tests/chown/preserve-root
@@ -38,13 +38,13 @@ chmod -R --preserve-root u+r / >> out 2>&1 && fail=1
 # since the symlink in question is not a command line argument.
 # Contrary to the above commands, these two should succeed.
 echo '==== test -RHh' >> out
-chown -RHh --preserve-root `id -u` d >> out 2>&1 || fail=1
-chgrp -RHh --preserve-root `id -g` d >> out 2>&1 || fail=1
+chown -RHh --preserve-root $(id -u) d >> out 2>&1 || fail=1
+chgrp -RHh --preserve-root $(id -g) d >> out 2>&1 || fail=1
 
 # These must fail.
 echo '==== test -RLh' >> out
-chown -RLh --preserve-root `id -u` d >> out 2>&1 && fail=1
-chgrp -RLh --preserve-root `id -g` d >> out 2>&1 && fail=1
+chown -RLh --preserve-root $(id -u) d >> out 2>&1 && fail=1
+chgrp -RLh --preserve-root $(id -g) d >> out 2>&1 && fail=1
 
 cat <<\EOF > exp || fail=1
 chown: it is dangerous to operate recursively on '/'
diff --git a/tests/chown/separator b/tests/chown/separator
index 6295fd78f..d4264a8c8 100755
--- a/tests/chown/separator
+++ b/tests/chown/separator
@@ -19,16 +19,16 @@
 . "${srcdir=.}/init.sh"; path_prepend_ ../src
 print_ver_ chown
 
-id_u=`id -u` || framework_failure_
+id_u=$(id -u) || framework_failure_
 test -n "$id_u" || framework_failure_
 
-id_un=`id -un` || framework_failure_
+id_un=$(id -un) || framework_failure_
 test -n "$id_un" || framework_failure_
 
-id_g=`id -g` || framework_failure_
+id_g=$(id -g) || framework_failure_
 test -n "$id_g" || framework_failure_
 
-id_gn=`id -gn` || framework_failure_
+id_gn=$(id -gn) || framework_failure_
 test -n "$id_gn" || framework_failure_
 
 # FreeBSD 6.x's getgrnam fails to look up a group name containing