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, 1 insertions, 1 deletions

diff --git a/tests/misc/sort-compress b/tests/misc/sort-compress
index 9a4b9f622..00dcf06ac 100755
--- a/tests/misc/sort-compress
+++ b/tests/misc/sort-compress
@@ -39,7 +39,7 @@ chmod +x gzip
 # Use a subshell and an exec to work around a bug in FreeBSD 5.0 /bin/sh.
 (
   # ash doesn't support "trap '' CHLD"; it knows only signal numbers.
-  sig=`"$abs_top_builddir/src/kill" -l CHLD 2>/dev/null` && trap '' $sig
+  sig=$("$abs_top_builddir/src/kill" -l CHLD 2>/dev/null) && trap '' $sig
 
   # This should force the use of child processes for "compression"
   PATH=.:$PATH exec sort -S 1k --compress-program=gzip in > /dev/null