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, 5 insertions, 5 deletions

diff --git a/tests/cp/acl b/tests/cp/acl
index 94b7af69b..26a7f420a 100755
--- a/tests/cp/acl
+++ b/tests/cp/acl
@@ -31,22 +31,22 @@ touch a/file || framework_failure_
 
 # Ensure that setfacl and getfacl work on this file system.
 skip=no
-acl1=`cd a && getfacl file` || skip=yes
+acl1=$(cd a && getfacl file) || skip=yes
 setfacl -m user:bin:rw- a/file 2> /dev/null || skip=yes
 test $skip = yes &&
   skip_ "'.' is not on a suitable file system for this test"
 
 # copy a file without preserving permissions
 cp a/file b/ || fail=1
-acl2=`cd b && getfacl file` || framework_failure_
+acl2=$(cd b && getfacl file) || framework_failure_
 test "$acl1" = "$acl2" || fail=1
 
 # Update with acl set above
-acl1=`cd a && getfacl file` || framework_failure_
+acl1=$(cd a && getfacl file) || framework_failure_
 
 # copy a file, preserving permissions
 cp -p a/file b/ || fail=1
-acl2=`cd b && getfacl file` || framework_failure_
+acl2=$(cd b && getfacl file) || framework_failure_
 test "$acl1" = "$acl2" || fail=1
 
 # copy a file, preserving permissions, with --attributes-only
@@ -54,7 +54,7 @@ echo > a/file || framework_failure_ # add some data
 test -s a/file || framework_failure_
 cp -p --attributes-only a/file b/ || fail=1
 test -s b/file && fail=1
-acl2=`cd b && getfacl file` || framework_failure_
+acl2=$(cd b && getfacl file) || framework_failure_
 test "$acl1" = "$acl2" || fail=1
 
 Exit $fail