[docs] Fixed outdated link with archive.is

The listed link is directing to an empty page, at least content-wise. I found the same page on archive.is and linked to that instead.

src/test/README.md

@@ -52,4 +52,4 @@ examine `uint256_tests.cpp`.
 
 For further reading, I found the following website to be helpful in
 explaining how the boost unit test framework works:
-[http://www.alittlemadness.com/2009/03/31/c-unit-testing-with-boosttest/](http://www.alittlemadness.com/2009/03/31/c-unit-testing-with-boosttest/).
+[http://www.alittlemadness.com/2009/03/31/c-unit-testing-with-boosttest/](http://archive.is/dRBGf).