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/chgrp/recurse b/tests/chgrp/recurse
index 9f9587bf3..95a910f6c 100755
--- a/tests/chgrp/recurse
+++ b/tests/chgrp/recurse
@@ -32,22 +32,22 @@ ln -s ../e d/s
 chgrp -R $g1 e/ee || fail=1
 # This should not should change the group of e/ee
 chgrp -R $g2 d
-set _ `ls -ln e/ee`; g=$5; test "$g" = $g1 || fail=1
+set _ $(ls -ln e/ee); g=$5; test "$g" = $g1 || fail=1
 # This must change the group of e/ee, since -L makes
 # chgrp traverse the symlink from d/s into e.
 chgrp -L -R $g2 d
-set _ `ls -ln e/ee`; g=$5; test "$g" = $g2 || fail=1
+set _ $(ls -ln e/ee); g=$5; test "$g" = $g2 || fail=1
 
 # This must *not* change the group of e/ee
 chgrp -H -R $g1 d
-set _ `ls -ln e/ee`; g=$5; test "$g" = $g2 || fail=1
+set _ $(ls -ln e/ee); g=$5; test "$g" = $g2 || fail=1
 
 ln -s d link
 
 # This shouldn't change the group of e/ee either.
 chgrp -H -R $g1 link || fail=1
-set _ `ls -ln e/ee`; g=$5; test "$g" = $g2 || fail=1
+set _ $(ls -ln e/ee); g=$5; test "$g" = $g2 || fail=1
 # But it *should* change d/dd.
-set _ `ls -ln d/dd`; g=$5; test "$g" = $g1 || fail=1
+set _ $(ls -ln d/dd); g=$5; test "$g" = $g1 || fail=1
 
 Exit $fail