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, 2 insertions, 2 deletions

diff --git a/tests/misc/close-stdout b/tests/misc/close-stdout
index 8af84c214..8584dfea4 100755
--- a/tests/misc/close-stdout
+++ b/tests/misc/close-stdout
@@ -49,7 +49,7 @@ if "$p/src/test" -w /dev/stdout >/dev/null &&
   rm -Rf tmpfile-?????? || fail=1
   mktemp tmpfile-XXXXXX >&- 2>/dev/null && fail=1
   mktemp tmpfile-XXXXXX -q >&- 2>/dev/null && fail=1
-  case `echo tmpfile-??????` in 'tmpfile-??????') ;; *) fail=1 ;; esac
+  case $(echo tmpfile-??????) in 'tmpfile-??????') ;; *) fail=1 ;; esac
 fi
 
 # Likewise for /dev/full, if /dev/full works.
@@ -59,7 +59,7 @@ if test -w /dev/full && test -c /dev/full; then
   rm -Rf tmpdir-?????? || fail=1
   mktemp -d tmpdir-XXXXXX >/dev/full 2>/dev/null && fail=1
   mktemp -d -q tmpdir-XXXXXX >/dev/full 2>/dev/null && fail=1
-  case `echo tmpfile-??????` in 'tmpfile-??????') ;; *) fail=1 ;; esac
+  case $(echo tmpfile-??????) in 'tmpfile-??????') ;; *) fail=1 ;; esac
 fi
 
 Exit $fail