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, 6 insertions, 6 deletions

diff --git a/tests/chown/basic b/tests/chown/basic
index 98145dac1..7c8951c83 100755
--- a/tests/chown/basic
+++ b/tests/chown/basic
@@ -25,12 +25,12 @@ touch f || framework_failure_
 chown -R --preserve-root 0:1 f
 
 # Make sure the owner and group are 0 and 1 respectively.
-set _ `ls -n f`; shift; test "$3:$4" = 0:1 || fail=1
+set _ $(ls -n f); shift; test "$3:$4" = 0:1 || fail=1
 
 # Make sure the correct diagnostic is output
 # Note we output a name even though an id was specified.
 chown -v --from=42 43 f > out || fail=1
-printf "ownership of 'f' retained as `id -nu`\n" > exp
+printf "ownership of 'f' retained as $(id -nu)\n" > exp
 compare exp out || fail=1
 
 # Ensure diagnostics work for non existent files.
@@ -41,19 +41,19 @@ compare exp out || fail=1
 chown --from=0:1 2:010 f || fail=1
 
 # And now they should be 2 and 10 respectively.
-set _ `ls -n f`; shift; test "$3:$4" = 2:10 || fail=1
+set _ $(ls -n f); shift; test "$3:$4" = 2:10 || fail=1
 
 ln -s f slink
 # Applying chown to a symlink with --no-dereference
 # should change only the link.
 chown --no-dereference 0:1 slink || fail=1
 # owner/group on the symlink should be set
-set _ `ls -n slink`; shift; test "$3:$4" = 0:1 || fail=1
+set _ $(ls -n slink); shift; test "$3:$4" = 0:1 || fail=1
 # owner/group on the referent should remain unchanged
-set _ `ls -n f`;     shift; test "$3:$4" = 2:10 || fail=1
+set _ $(ls -n f);     shift; test "$3:$4" = 2:10 || fail=1
 
 chown --no-dereference --from=0:1 2:010 slink || fail=1
 # owner/group on the symlink should be changed
-set _ `ls -n slink`; shift; test "$3:$4" = 2:10 || fail=1
+set _ $(ls -n slink); shift; test "$3:$4" = 2:10 || fail=1
 
 Exit $fail