DOMParser: Tweak tests for interface of returned document

See https://github.com/whatwg/html/issues/11372. This matches all
browsers.

domparsing/DOMParser-parseFromString-html.html

diff --git a/domparsing/DOMParser-parseFromString-html.html b/domparsing/DOMParser-parseFromString-html.html
index 86e516d..9834d3f 100644
--- a/domparsing/DOMParser-parseFromString-html.html
+++ b/domparsing/DOMParser-parseFromString-html.html
@@ -25,6 +25,10 @@
 }, 'Parsing of id attribute');
 
 test(function() {
+    assert_true(doc instanceof HTMLDocument);
+}, 'should be an HTMLDocument');
+
+test(function() {
     assert_equals(doc.contentType, "text/html")
 }, 'contentType');
 

domparsing/DOMParser-parseFromString-xml.html

diff --git a/domparsing/DOMParser-parseFromString-xml.html b/domparsing/DOMParser-parseFromString-xml.html
index b07bb3d..c7f6b89 100644
--- a/domparsing/DOMParser-parseFromString-xml.html
+++ b/domparsing/DOMParser-parseFromString-xml.html
@@ -33,7 +33,7 @@
   test(function() {
     var p = new DOMParser();
     var doc = p.parseFromString("<foo/>", type);
-    assert_false(doc instanceof XMLDocument, "Should not be XMLDocument");
+    assert_true(doc instanceof XMLDocument, "Should be XMLDocument");
   }, "XMLDocument interface for correctly parsed document with type " + type);
 
   test(function() {
@@ -48,7 +48,7 @@
   test(function() {
     var p = new DOMParser();
     var doc = p.parseFromString("<foo>", type);
-    assert_false(doc instanceof XMLDocument, "Should not be XMLDocument");
+    assert_true(doc instanceof XMLDocument, "Should be XMLDocument");
   }, "XMLDocument interface for incorrectly parsed document with type " + type);
 
   test(function() {