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/misc/date-sec b/tests/misc/date-sec
index 817e9243f..df77fe7bd 100755
--- a/tests/misc/date-sec
+++ b/tests/misc/date-sec
@@ -24,13 +24,13 @@ print_ver_ date
 
 
 # It would be easier simply to sleep for two seconds between two runs
-# of `date --date="21:04 +0100" +%S` and ensure that both outputs
+# of $(date --date="21:04 +0100" +%S) and ensure that both outputs
 # are '00', but I prefer not to sleep unconditionally.  'make check'
 # takes long enough as it is.
 
 n=0
 # See if the current number of seconds is '00' or just before.
-s=`date +%S`
+s=$(date +%S)
 case "$s" in
   58) n=3;;
   59) n=2;;
@@ -40,7 +40,7 @@ esac
 # If necessary, wait for the system clock to pass the minute mark.
 test $n = 0 || sleep $n
 
-s=`date --date="21:04 +0100" +%S`
+s=$(date --date="21:04 +0100" +%S)
 case "$s" in
   00) ;;
   *) fail=1;;