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/touch/empty-file b/tests/touch/empty-file
index cd679c41e..a150bd58e 100755
--- a/tests/touch/empty-file
+++ b/tests/touch/empty-file
@@ -48,7 +48,7 @@ echo sleeping for $SLEEP_SECONDS seconds...
 sleep $SLEEP_SECONDS
 for d in $TOUCH_DIR_LIST; do
   touch $d/a || fail=1
-  set x `ls -t $d/a $d/b`
+  set x $(ls -t $d/a $d/b)
   test "$*" = "x $d/a $d/b" || fail=1
 done
 
@@ -56,11 +56,11 @@ echo sleeping for $SLEEP_SECONDS seconds...
 sleep $SLEEP_SECONDS
 for d in $TOUCH_DIR_LIST; do
   touch $d/b
-  set x `ls -t $d/a $d/b`
+  set x $(ls -t $d/a $d/b)
   test "$*" = "x $d/b $d/a" || fail=1
 
   if touch - 1< $d/c 2> /dev/null; then
-    set x `ls -t $d/a $d/c`
+    set x $(ls -t $d/a $d/c)
     test "$*" = "x $d/c $d/a" || fail=1
   fi