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, 9 insertions, 9 deletions

diff --git a/tests/misc/env b/tests/misc/env
index ec9f0af1e..6f8c064ce 100755
--- a/tests/misc/env
+++ b/tests/misc/env
@@ -95,8 +95,8 @@ echo pass
 EOF
 chmod +x unlikely_name/also_unlikely || framework_failure_
 env also_unlikely && fail=1
-test x`PATH=$PATH:unlikely_name env also_unlikely` = xpass || fail=1
-test x`env PATH="$PATH":unlikely_name also_unlikely` = xpass || fail=1
+test x$(PATH=$PATH:unlikely_name env also_unlikely) = xpass || fail=1
+test x$(env PATH="$PATH":unlikely_name also_unlikely) = xpass || fail=1
 
 # Explicitly put . on the PATH for the rest of this test.
 PATH=$PATH:
@@ -108,15 +108,15 @@ export PATH
 # Avoid the issue by using an executable rather than a script.
 # Test -u, rather than -i, to minimize PATH problems.
 ln -s "$abs_top_builddir/src/echo" ./-u || framework_failure_
-case `env -u echo echo good` in
+case $(env -u echo echo good) in
   good) ;;
   *) fail=1 ;;
 esac
-case `env -u echo -- echo good` in
+case $(env -u echo -- echo good) in
   good) ;;
   *) fail=1 ;;
 esac
-case `env -- -u pass` in
+case $(env -- -u pass) in
   pass) ;;
   *) fail=1 ;;
 esac
@@ -125,7 +125,7 @@ esac
 env a=b -- true
 test $? = 127 || fail=1
 ln -s "$abs_top_builddir/src/echo" ./-- || framework_failure_
-case `env a=b -- true || echo fail` in
+case $(env a=b -- true || echo fail) in
   true) ;;
   *) fail=1 ;;
 esac
@@ -136,9 +136,9 @@ cat <<EOF >./c=d || framework_failure_
 echo pass
 EOF
 chmod +x c=d || framework_failure_
-test "x`env c=d echo fail`" = xfail || fail=1
-test "x`env -- c=d echo fail`" = xfail || fail=1
-test "x`env ./c=d echo fail`" = xfail || fail=1
+test "x$(env c=d echo fail)" = xfail || fail=1
+test "x$(env -- c=d echo fail)" = xfail || fail=1
+test "x$(env ./c=d echo fail)" = xfail || fail=1
 test "x$(env sh -c 'exec "$@"' sh c=d echo fail)" = xpass || fail=1
 test "x$(sh -c '\c=d echo fail')" = xpass && #dash 0.5.4 fails so check first
   { test "x$(env sh -c '\c=d echo fail')" = xpass || fail=1; }