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, 9 insertions, 9 deletions

diff --git a/tests/chgrp/deref b/tests/chgrp/deref
index 18946b1fa..4c186ee82 100755
--- a/tests/chgrp/deref
+++ b/tests/chgrp/deref
@@ -28,7 +28,7 @@ touch f
 ln -s f symlink
 
 chgrp -h $g2 symlink 2> /dev/null
-set _ `ls -ln symlink`
+set _ $(ls -ln symlink)
 g=$5
 test "$g" = $g2 ||
   skip_ "your system doesn't support changing the owner or group" \
@@ -36,25 +36,25 @@ test "$g" = $g2 ||
 
 
 chgrp $g1 f
-set _ `ls -ln f`; g=$5; test "$g" = $g1 || fail=1
+set _ $(ls -ln f); g=$5; test "$g" = $g1 || fail=1
 
 chgrp -h $g2 symlink || fail=1
-set _ `ls -ln f`; g=$5; test "$g" = $g1 || fail=1
-set _ `ls -ln symlink`; g=$5; test "$g" = $g2 || fail=1
+set _ $(ls -ln f); g=$5; test "$g" = $g1 || fail=1
+set _ $(ls -ln symlink); g=$5; test "$g" = $g2 || fail=1
 
 # This should not change the group of f.
 chgrp -h $g2 symlink || fail=1
-set _ `ls -ln f`; g=$5; test "$g" = $g1 || fail=1
-set _ `ls -ln symlink`; g=$5; test "$g" = $g2 || fail=1
+set _ $(ls -ln f); g=$5; test "$g" = $g1 || fail=1
+set _ $(ls -ln symlink); g=$5; test "$g" = $g2 || fail=1
 
 chgrp $g2 f
-set _ `ls -ln f`; g=$5; test "$g" = $g2 || fail=1
+set _ $(ls -ln f); g=$5; test "$g" = $g2 || fail=1
 
 # This *should* change the group of f.
 # Though note that the diagnostic you'd get with -c is misleading in that
 # it says the 'group of 'symlink'' has been changed.
 chgrp --dereference $g1 symlink
-set _ `ls -ln f`; g=$5; test "$g" = $g1 || fail=1
-set _ `ls -ln symlink`; g=$5; test "$g" = $g2 || fail=1
+set _ $(ls -ln f); g=$5; test "$g" = $g1 || fail=1
+set _ $(ls -ln symlink); g=$5; test "$g" = $g2 || fail=1
 
 Exit $fail