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/mv/update b/tests/mv/update
index d002a183b..f0beb4076 100755
--- a/tests/mv/update
+++ b/tests/mv/update
@@ -31,15 +31,15 @@ for interactive in '' -i; do
     # mistakenly elicit a prompt.
     $cp_or_mv $interactive --update old new < /dev/null > out 2>&1 || fail=1
     test -s out && fail=1
-    case "`cat new`" in new) ;; *) fail=1 ;; esac
-    case "`cat old`" in old) ;; *) fail=1 ;; esac
+    case "$(cat new)" in new) ;; *) fail=1 ;; esac
+    case "$(cat old)" in old) ;; *) fail=1 ;; esac
   done
 done
 
 # This will actually perform the rename.
 mv --update new old || fail=1
 test -f new && fail=1
-case "`cat old`" in new) ;; *) fail=1 ;; esac
+case "$(cat old)" in new) ;; *) fail=1 ;; esac
 
 # Restore initial conditions.
 echo old > old || fail=1
@@ -48,7 +48,7 @@ echo new > new || fail=1
 
 # This will actually perform the copy.
 cp --update new old || fail=1
-case "`cat old`" in new) ;; *) fail=1 ;; esac
-case "`cat new`" in new) ;; *) fail=1 ;; esac
+case "$(cat old)" in new) ;; *) fail=1 ;; esac
+case "$(cat new)" in new) ;; *) fail=1 ;; esac
 
 Exit $fail