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/cp/cp-parents b/tests/cp/cp-parents
index 85fccd63b..b325c722b 100755
--- a/tests/cp/cp-parents
+++ b/tests/cp/cp-parents
@@ -49,10 +49,10 @@ test -d d/f && fail=1
 chmod go=w d/a || framework_failure_
 cp -a --parents d/a/b/c e || fail=1
 cp -a --parents sym/b/c g || fail=1
-p=`ls -ld e/d|cut -b-10`; case $p in drwxr-xr-x);; *) fail=1;; esac
-p=`ls -ld e/d/a|cut -b-10`; case $p in drwx-w--w-);; *) fail=1;; esac
-p=`ls -ld g/sym|cut -b-10`; case $p in drwx-w--w-);; *) fail=1;; esac
-p=`ls -ld e/d/a/b/c|cut -b-10`; case $p in drwxr-xr-x);; *) fail=1;; esac
-p=`ls -ld g/sym/b/c|cut -b-10`; case $p in drwxr-xr-x);; *) fail=1;; esac
+p=$(ls -ld e/d|cut -b-10); case $p in drwxr-xr-x);; *) fail=1;; esac
+p=$(ls -ld e/d/a|cut -b-10); case $p in drwx-w--w-);; *) fail=1;; esac
+p=$(ls -ld g/sym|cut -b-10); case $p in drwx-w--w-);; *) fail=1;; esac
+p=$(ls -ld e/d/a/b/c|cut -b-10); case $p in drwxr-xr-x);; *) fail=1;; esac
+p=$(ls -ld g/sym/b/c|cut -b-10); case $p in drwxr-xr-x);; *) fail=1;; esac
 
 Exit $fail