Add Request.abort() test

This patch:
- introduces StaticServer for the testing purposes
- switches tests from navigating to file:// to http://
- adds a test to cover Request.abort() functionality

References #1

test/assets/one-style.css

@@ -0,0 +1,3 @@
+body {
+    background-color: pink;
+}

test/assets/one-style.html

@@ -0,0 +1,2 @@
+<link rel='stylesheet' href='./one-style.css'>
+<div>hello, world!</div>