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/du/long-from-unreadable b/tests/du/long-from-unreadable
index 10864e00e..7fc129c17 100755
--- a/tests/du/long-from-unreadable
+++ b/tests/du/long-from-unreadable
@@ -36,11 +36,11 @@ if test ! -d $proc_file; then
   skip_ 'This test would fail, since your system lacks /proc support.'
 fi
 
-dir=`printf '%200s\n' ' '|tr ' ' x`
+dir=$(printf '%200s\n' ' '|tr ' ' x)
 
 # Construct a hierarchy containing a relative file with a name
 # longer than PATH_MAX.
-# for i in `seq 52`; do
+# for i in $(seq 52); do
 #   mkdir $dir || framework_failure_
 #   cd $dir || framework_failure_
 # done
@@ -51,7 +51,7 @@ dir=`printf '%200s\n' ' '|tr ' ' x`
 #     cannot access parent directories:
 # (all on one line).
 
-cwd=`pwd`
+cwd=$(pwd)
 # Use perl instead:
 : ${PERL=perl}
 $PERL \