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/mv/acl b/tests/mv/acl
index 382752a33..2354ed967 100755
--- a/tests/mv/acl
+++ b/tests/mv/acl
@@ -38,30 +38,30 @@ skip_partition=none
 setfacl -m user:bin:rw- file 2> /dev/null || skip_partition=.
 # And on the destination file system.
 setfacl -m user:bin:rw- $t1 || skip_partition=$other_partition_tmpdir
-acl1=`getfacl file` || skip_partition=.
+acl1=$(getfacl file) || skip_partition=.
 
 test $skip_partition != none &&
   skip_ "'$skip_partition' is not on a suitable file system for this test"
 
 # move the access acl of a file
 mv file "$other_partition_tmpdir" || fail=1
-acl2=`cd "$other_partition_tmpdir" && getfacl file` || framework_failure_
+acl2=$(cd "$other_partition_tmpdir" && getfacl file) || framework_failure_
 test "$acl1" = "$acl2" || fail=1
 
 # move the access acl of a directory
 mkdir dir || framework_failure_
 setfacl -m user:bin:rw- dir || framework_failure_
-acl1=`getfacl dir` || framework_failure_
+acl1=$(getfacl dir) || framework_failure_
 mv dir "$other_partition_tmpdir" || fail=1
-acl2=`cd "$other_partition_tmpdir" && getfacl dir` || framework_failure_
+acl2=$(cd "$other_partition_tmpdir" && getfacl dir) || framework_failure_
 test "$acl1" = "$acl2" || fail=1
 
 # move the default acl of a directory
 mkdir dir2 || framework_failure_
 setfacl -d -m user:bin:rw- dir2 || framework_failure_
-acl1=`getfacl dir2` || framework_failure_
+acl1=$(getfacl dir2) || framework_failure_
 mv dir2 "$other_partition_tmpdir" || fail=1
-acl2=`cd "$other_partition_tmpdir" && getfacl dir2` || framework_failure_
+acl2=$(cd "$other_partition_tmpdir" && getfacl dir2) || framework_failure_
 test "$acl1" = "$acl2" || fail=1
 
 Exit $fail