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

2 files changed, 2 insertions, 2 deletions

diff --git a/tests/tail-2/assert b/tests/tail-2/assert
index 37b9afbcf..043e25937 100755
--- a/tests/tail-2/assert
+++ b/tests/tail-2/assert
@@ -51,7 +51,7 @@ mv f foo
 # echo waiting....
 wait
 
-case "`cat err`" in
+case "$(cat err)" in
   *$ok) ;;
   *) fail=1;;
 esac
diff --git a/tests/tail-2/assert-2 b/tests/tail-2/assert-2
index e6c95f824..d94d3d8e2 100755
--- a/tests/tail-2/assert-2
+++ b/tests/tail-2/assert-2
@@ -39,7 +39,7 @@ mv f foo
 # echo waiting....
 wait
 
-case "`cat err`" in
+case "$(cat err)" in
   *$ok) ;;
   *) fail=1;;
 esac