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/mv/leak-fd b/tests/mv/leak-fd
index 532bdb082..d349a3f11 100755
--- a/tests/mv/leak-fd
+++ b/tests/mv/leak-fd
@@ -32,16 +32,16 @@ b="0 1 2 3 4 5 6 7 8 9
 a b c d e f g h i j k l m n o p q r s t u v w x y z
 _A _B _C _D _E _F _G _H _I _J _K _L _M _N _O _P _Q _R _S _T _U _V _W _X _Y _Z"
 
-for i in `echo $b`; do
+for i in $(echo $b); do
   echo $i
   for j in $b; do
     echo $i$j
   done
 done > .dirs
-mkdir `cat .dirs` || framework_failure_
+mkdir $(cat .dirs) || framework_failure_
 sed 's,$,/f,' .dirs | xargs touch
 
-last_file=`tail -n1 .dirs`/f
+last_file=$(tail -n1 .dirs)/f
 test -f $last_file || framework_failure_