Migrate skip checks to --EXTENSIONS--, p2

For rationale, see php#6787

Make extension checks lowercase, add a special case for opcache
that has internal name not matching .so filename.

Extensions migrated in part 2:
* dom
* exif
* fileinfo
* ffi

Author: MaxSem

ext/dom/tests/DOM4_ChildNode_wrong_document.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMChildNode::after(), before, replaceWith with DOMNode from wrong document throws exception
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");

ext/dom/tests/DOM4_DOMNode_ElementSiblings.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode: Element Siblings
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");

ext/dom/tests/DOM4_DOMNode_after.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode::after()
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");

ext/dom/tests/DOM4_DOMNode_after_ns.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode::after() with namespace
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");

ext/dom/tests/DOM4_DOMNode_append_ns.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode::append() with namespace
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");

ext/dom/tests/DOM4_DOMNode_before.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode::before()
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");

ext/dom/tests/DOM4_DOMNode_before_ns.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode::before() with namespace
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");

ext/dom/tests/DOM4_DOMNode_prepend_ns.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode::prepend() with namespace
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");

ext/dom/tests/DOM4_DOMNode_remove.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode::remove()
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php
 require_once("dom_test.inc");
@@ -27,4 +27,3 @@ Num Children: 1
 Node Content: second
 
 NULL
-

ext/dom/tests/DOM4_DOMNode_removeDanglingElement.phpt

@@ -1,7 +1,7 @@
 --TEST--
 DOMNode::remove() dangling element
---SKIPIF--
-<?php require_once('skipif.inc'); ?>
+--EXTENSIONS--
+dom
 --FILE--
 <?php