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, 5 insertions, 5 deletions

diff --git a/tests/misc/stty-row-col b/tests/misc/stty-row-col
index bcbc7bc64..d7151d5b7 100755
--- a/tests/misc/stty-row-col
+++ b/tests/misc/stty-row-col
@@ -51,7 +51,7 @@ NA LAST NA
 '
 set $tests
 
-saved_size=`stty size` && test -n "$saved_size" \
+saved_size=$(stty size) && test -n "$saved_size" \
   || skip_ "can't get window size"
 
 # Linux virtual consoles issue an error if you
@@ -66,16 +66,16 @@ fi
 while :; do
   test_name=$1
   args=$2
-  expected_result="`echo $3|tr _ ' '`"
+  expected_result="$(echo $3|tr _ ' ')"
   test "$args" = empty && args=''
   test "x$args" = xLAST && break
-  args=`echo x$args|tr _ ' '|sed 's/^x//'`
+  args=$(echo x$args|tr _ ' '|sed 's/^x//')
   if test "$VERBOSE" = yes; then
-    # echo "testing \`stty $args; stty size\` = $expected_result ..."
+    # echo "testing \$(stty $args; stty size\) = $expected_result ..."
     echo "test $test_name... " | tr -d '\n'
   fi
   stty $args || exit 1
-  test x"`stty size 2> /dev/null`" = "x$expected_result" \
+  test x"$(stty size 2> /dev/null)" = "x$expected_result" \
     && ok=ok || ok=FAIL fail=1
   test "$VERBOSE" = yes && echo $ok
   shift; shift; shift