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, 6 insertions, 6 deletions

diff --git a/tests/ls/stat-vs-dirent b/tests/ls/stat-vs-dirent
index 60d96220a..1dd6e6621 100755
--- a/tests/ls/stat-vs-dirent
+++ b/tests/ls/stat-vs-dirent
@@ -20,20 +20,20 @@
 print_ver_ ls
 
 
-root_dev_ino=`stat --format=%d-%i /`
-t=`pwd`
+root_dev_ino=$(stat --format=%d-%i /)
+t=$(pwd)
 while :; do
   ls -i1 "$t" > tmp
   if test $? = 0; then
     # Extract the inode number from the first line of output from ls -i1.
     # This value comes from dirent.d_ino, on systems with d_ino support.
-    d_ino=`sed -n '1s/^ *\([0-9][0-9]*\) .*/\1/p;q' tmp`
+    d_ino=$(sed -n '1s/^ *\([0-9][0-9]*\) .*/\1/p;q' tmp)
 
     # Extract the name of the corresponding directory entry.
-    file=`sed -n '1s/^ *[0-9][0-9]*  *//p;q' tmp`
+    file=$(sed -n '1s/^ *[0-9][0-9]*  *//p;q' tmp)
 
     # Get its inode number (stat.st_ino) via stat(1)'s call to lstat.
-    st_ino=`stat --format=%i "$t/$file"`
+    st_ino=$(stat --format=%i "$t/$file")
 
     # Make sure that they are the same.
     # We know from experience that there may be mismatches on some
@@ -53,7 +53,7 @@ while :; do
   fi
 
   t=$(cd "$t/.."; pwd)
-  dev_ino=`stat --format=%d-%i "$t"`
+  dev_ino=$(stat --format=%d-%i "$t")
   test $dev_ino = $root_dev_ino && break
 done