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/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