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, 2 insertions, 2 deletions

diff --git a/tests/install/basic-1 b/tests/install/basic-1
index 722a947c0..aa80fcf33 100755
--- a/tests/install/basic-1
+++ b/tests/install/basic-1
@@ -57,7 +57,7 @@ ginstall -s -c -m 555 $dd $dir || fail=1
 test -f $dd || fail=1
 
 # Make sure that the destination file has the requested permissions.
-mode=`ls -l $dir/$dd|cut -b-10`
+mode=$(ls -l $dir/$dd|cut -b-10)
 test "$mode" = -r-xr-xr-x || fail=1
 
 # These failed in coreutils CVS from 2004-06-25 to 2004-08-11.
@@ -72,7 +72,7 @@ test -d newdir3 || fail=1
 # This fails because mkdir-p.c's make_dir_parents fails to return to its
 # initial working directory ($iwd) after creating the first argument, and
 # hence cannot do anything meaningful with the following relative-named dirs.
-iwd=`pwd`
+iwd=$(pwd)
 mkdir sub || fail=1
 (cd sub &&
  chmod 0 . &&