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/ls/inode b/tests/ls/inode
index b5c881e27..8bf5ba153 100755
--- a/tests/ls/inode
+++ b/tests/ls/inode
@@ -25,17 +25,17 @@ ln -s f slink || framework_failure_
 
 # When listed explicitly:
 
-set x `ls -Ci f slink`; shift
+set x $(ls -Ci f slink); shift
 test $# = 4 || fail=1
 # The inode numbers should differ.
 test "$1" != "$3" || fail=1
 
-set x `ls -CLi f slink`; shift
+set x $(ls -CLi f slink); shift
 test $# = 4 || fail=1
 # With -L, they must be the same.
 test "$1" = "$3" || fail=1
 
-set x `ls -CHi f slink`; shift
+set x $(ls -CHi f slink); shift
 test $# = 4 || fail=1
 # With -H, they must be the same, too, from the command line.
 # Note that POSIX says -H must make ls dereference only
@@ -47,17 +47,17 @@ test "$1" = "$3" || fail=1
 
 # When listed from a directory:
 
-set x `ls -Ci`; shift
+set x $(ls -Ci); shift
 test $# = 4 || fail=1
 # The inode numbers should differ.
 test "$1" != "$3" || fail=1
 
-set x `ls -CLi`; shift
+set x $(ls -CLi); shift
 test $# = 4 || fail=1
 # With -L, they must be the same.
 test "$1" = "$3" || fail=1
 
-set x `ls -CHi`; shift
+set x $(ls -CHi); shift
 test $# = 4 || fail=1
 # With -H, they must be different from inside a directory.
 test "$1" != "$3" || fail=1