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, 3 insertions, 3 deletions

diff --git a/tests/du/deref b/tests/du/deref
index bf2ead4d7..90b765039 100755
--- a/tests/du/deref
+++ b/tests/du/deref
@@ -39,9 +39,9 @@ du -L dangle > /dev/null 2>&1 && fail=1
 # du -L used to mess up, either by counting the symlink's disk space itself
 # (-L should follow symlinks, not count their space)
 # or (briefly in July 2010) by omitting the entry for "a".
-du_L_output=`du -L a` || fail=1
-du_lL_output=`du -lL a` || fail=1
-du_x_output=`du --exclude=dotdot a` || fail=1
+du_L_output=$(du -L a) || fail=1
+du_lL_output=$(du -lL a) || fail=1
+du_x_output=$(du --exclude=dotdot a) || fail=1
 test "X$du_L_output" = "X$du_x_output" || fail=1
 test "X$du_lL_output" = "X$du_x_output" || fail=1