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

4 files changed, 9 insertions, 9 deletions

diff --git a/tests/dd/bytes b/tests/dd/bytes
index 15755aae3..2d40f0641 100755
--- a/tests/dd/bytes
+++ b/tests/dd/bytes
@@ -21,7 +21,7 @@ print_ver_ dd
 # count_bytes
 echo 0123456789abcdefghijklm > in || framework_failure_
 dd count=14 conv=swab iflag=count_bytes < in > out 2> /dev/null || fail=1
-case `cat out` in
+case $(cat out) in
  1032547698badc) ;;
  *) fail=1 ;;
 esac
@@ -29,7 +29,7 @@ esac
 # skip_bytes
 echo 0123456789abcdefghijklm > in || framework_failure_
 dd skip=10 iflag=skip_bytes < in > out 2> /dev/null || fail=1
-case `cat out` in
+case $(cat out) in
  abcdefghijklm) ;;
  *) fail=1 ;;
 esac
@@ -37,7 +37,7 @@ esac
 # skip records and bytes from pipe
 echo 0123456789abcdefghijklm |
  dd skip=10 bs=2 iflag=skip_bytes > out 2> /dev/null || fail=1
-case `cat out` in
+case $(cat out) in
  abcdefghijklm) ;;
  *) fail=1 ;;
 esac
diff --git a/tests/dd/misc b/tests/dd/misc
index aad04dcb1..52a5b2081 100755
--- a/tests/dd/misc
+++ b/tests/dd/misc
@@ -57,10 +57,10 @@ if dd iflag=directory if=. count=0 2> /dev/null; then
   dd iflag=directory count=0 <$tmp_in 2> /dev/null && fail=1
 fi
 
-old_ls=`ls -u --full-time $tmp_in`
+old_ls=$(ls -u --full-time $tmp_in)
 sleep 1
 if dd iflag=noatime if=$tmp_in of=$tmp_out 2> /dev/null; then
-  new_ls=`ls -u --full-time $tmp_in`
+  new_ls=$(ls -u --full-time $tmp_in)
   if test "x$old_ls" != "x$new_ls"; then
     cat >&2 <<EOF
 =================================================================
@@ -80,7 +80,7 @@ if dd oflag=nolinks if=$tmp_in of=$tmp_out 2> /dev/null; then
   dd oflag=nolinks < $tmp_in > $tmp_out 2>&1 || fail=1
 fi
 
-outbytes=`echo x | dd bs=3 ibs=10 obs=10 conv=sync 2>/dev/null | wc -c`
+outbytes=$(echo x | dd bs=3 ibs=10 obs=10 conv=sync 2>/dev/null | wc -c)
 test "$outbytes" -eq 3 || fail=1
 
 # A delay is required to trigger a failure.
diff --git a/tests/dd/not-rewound b/tests/dd/not-rewound
index 284dd4a1a..5364f907f 100755
--- a/tests/dd/not-rewound
+++ b/tests/dd/not-rewound
@@ -23,7 +23,7 @@ print_ver_ dd
 
 echo abcde > in
 (dd skip=1 count=1 bs=1; dd skip=1 bs=1) < in > out 2> /dev/null || fail=1
-case `cat out` in
+case $(cat out) in
   bde) ;;
   *) fail=1 ;;
 esac
diff --git a/tests/dd/skip-seek2 b/tests/dd/skip-seek2
index e1d3edb41..7f5e0560f 100755
--- a/tests/dd/skip-seek2
+++ b/tests/dd/skip-seek2
@@ -23,14 +23,14 @@ print_ver_ dd
 
 echo LA:3456789abcdef > in || fail=1
 (dd bs=1 skip=3 count=0 && dd bs=5) < in > out 2> /dev/null || fail=1
-case `cat out` in
+case $(cat out) in
   3456789abcdef) ;;
   *) fail=1 ;;
 esac
 
 echo LA:3456789abcdef > in || fail=1
 (dd bs=1 skip=3 count=0 && dd bs=5 count=2) < in > out 2> /dev/null || fail=1
-case `cat out` in
+case $(cat out) in
   3456789abc) ;;
   *) fail=1 ;;
 esac