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, 3 insertions, 3 deletions

diff --git a/tests/misc/stty-invalid b/tests/misc/stty-invalid
index 2be98437e..3b035ecef 100755
--- a/tests/misc/stty-invalid
+++ b/tests/misc/stty-invalid
@@ -22,7 +22,7 @@ require_controlling_input_terminal_
 trap '' TTOU # Ignore SIGTTOU
 
 
-saved_state=`stty -g` || fail=1
+saved_state=$(stty -g) || fail=1
 stty $saved_state || fail=1
 
 # Before coreutils-6.9.90, if stty were given an argument with 35 colons
@@ -34,8 +34,8 @@ stty $saved_state || fail=1
 # For each of the following, with coreutils-6.9 and earlier,
 # stty would fail to diagnose the error on at least Solaris 10.
 hex_2_64=10000000000000000
-stty `echo $saved_state |sed 's/^[^:]*:/'$hex_2_64:/`     2>/dev/null && fail=1
-stty `echo $saved_state |sed 's/:[0-9a-f]*$/:'$hex_2_64/` 2>/dev/null && fail=1
+stty $(echo $saved_state |sed 's/^[^:]*:/'$hex_2_64:/)     2>/dev/null && fail=1
+stty $(echo $saved_state |sed 's/:[0-9a-f]*$/:'$hex_2_64/) 2>/dev/null && fail=1
 
 # Just in case either of the above mistakenly succeeds (and changes
 # the state of our tty), try to restore the initial state.