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, 7 insertions, 7 deletions

diff --git a/tests/misc/nice b/tests/misc/nice
index cbc135fda..ac8ea04ea 100755
--- a/tests/misc/nice
+++ b/tests/misc/nice
@@ -45,7 +45,7 @@ NA LAST NA
 set $tests
 
 # Require that this test be run at 'nice' level 0.
-niceness=`nice`
+niceness=$(nice)
 if test "$niceness" = 0; then
     : ok
 else
@@ -58,19 +58,19 @@ while :; do
   expected_result=$3
   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 \`nice $args nice\` = $expected_result ..."
+    #echo "testing \$(nice $args nice\) = $expected_result ..."
     echo "test $test_name... " | tr -d '\n'
   fi
-  test x`nice $args nice 2> /dev/null` = x$expected_result \
+  test x$(nice $args nice 2> /dev/null) = x$expected_result \
     && ok=ok || ok=FAIL fail=1
   test "$VERBOSE" = yes && echo $ok
   shift; shift; shift
 done
 
 # Test negative niceness - command must be run whether or not change happens.
-if test x`nice -n -1 nice 2> /dev/null` = x0 ; then
+if test x$(nice -n -1 nice 2> /dev/null) = x0 ; then
   # unprivileged user - warn about failure to change
   nice -n -1 true 2> err || fail=1
   test -s err || fail=1
@@ -87,8 +87,8 @@ else
   # superuser - change succeeds
   nice -n -1 nice 2> err || fail=1
   test -s err && fail=1
-  test x`nice -n -1 nice` = x-1 || fail=1
-  test x`nice --1 nice` = x-1 || fail=1
+  test x$(nice -n -1 nice) = x-1 || fail=1
+  test x$(nice --1 nice) = x-1 || fail=1
 fi
 
 Exit $fail