correct default expectation for existing mrow test (#22226)

* Update tabindex-001.html

Attempting to match defaults defined in whatwg/html#5248
definitely needs scrutiny..

* Update tabindex-001.html

The default `.tabIndex` of an mrow should be 0 regardless of when it has an href or not, as a linkable element and matching historical (oddity) of other linkable things in HTML/SVG

mathml/relations/html5-tree/tabindex-001.html

@@ -12,13 +12,13 @@
   window.addEventListener("load", function() {
       test(() => {
           const mrow = document.getElementById('mrow');
-          assert_equals(mrow.tabIndex, -1, "no attribute");
+          assert_equals(mrow.tabIndex, 0, "no attribute");
           mrow.setAttribute("tabindex", "invalid");
           assert_equals(mrow.getAttribute("tabindex"), "invalid");
-          assert_equals(mrow.tabIndex, -1, "invalid");
+          assert_equals(mrow.tabIndex, 0, "invalid");
           mrow.setAttribute("tabindex", "9999999999");
           assert_equals(mrow.getAttribute("tabindex"), "9999999999");
-          assert_equals(mrow.tabIndex, -1, "too large integer");
+          assert_equals(mrow.tabIndex, 0, "too large integer");
       }, "default and invalid values on mrow");
       test(() => {
           const mrowLink = document.getElementById('mrow-link');