Rename fetch layout tests.

LayoutTests/http/tests/fetch/window/body-mixin.html

Index: LayoutTests/http/tests/fetch/window/body-mixin.html
diff --git a/LayoutTests/http/tests/fetch/window/response.html b/LayoutTests/http/tests/fetch/window/body-mixin.html
similarity index 94%
copy from LayoutTests/http/tests/fetch/window/response.html
copy to LayoutTests/http/tests/fetch/window/body-mixin.html
index 96fffcfff30f0454489388191427419051635017..7b422e9d0bf78b21b45fc64b2408f05e3269811a 100644
--- a/LayoutTests/http/tests/fetch/window/response.html
+++ b/LayoutTests/http/tests/fetch/window/body-mixin.html
@@ -12,7 +12,7 @@
 <script>
 function start(t) {
   var script = document.createElement('script');
-  script.src = '../script-tests/response.js?';
+  script.src = '../script-tests/body-mixin.js?';
   script.addEventListener(
     'error',
     function() { unreached_rejection(t); });