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/misc/od-multiple-t b/tests/misc/od-multiple-t
index 1f607bc87..049273cdf 100755
--- a/tests/misc/od-multiple-t
+++ b/tests/misc/od-multiple-t
@@ -23,17 +23,17 @@ print_ver_ od
 # Choose 48 bytes for the input, as that is lcm for 1, 2, 4, 8, 12, 16;
 # we don't anticipate any other native object size on modern hardware.
 seq 19 > in || framework_failure_
-test `wc -c < in` -eq 48 || framework_failure_
+test $(wc -c < in) -eq 48 || framework_failure_
 
 
 list='a c dC dS dI dL oC oS oI oL uC uS uI uL xC xS xI xL fF fD fL'
 for format1 in $list; do
   for format2 in $list; do
     od -An -t${format1}z -t${format2}z in > out-raw || fail=1
-    linewidth=`head -n1 out-raw | wc -c`
-    linecount=`wc -l < out-raw`
-    echo $format1 $format2 `wc -c < out-raw` >> out
-    echo $format1 $format2 `expr $linewidth '*' $linecount` >> exp
+    linewidth=$(head -n1 out-raw | wc -c)
+    linecount=$(wc -l < out-raw)
+    echo $format1 $format2 $(wc -c < out-raw) >> out
+    echo $format1 $format2 $(expr $linewidth '*' $linecount) >> exp
   done
 done