[backport#11804] [docs] Fixed outdated link with archive.is

Summary:
bf20a7d [docs] Fixed outdated link with archive.is (Tim Shimmin)

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Backport of Core [[bitcoin/bitcoin#11804 | PR11804]]

Test Plan: read it

Reviewers: #bitcoin_abc, PiRK, Fabien

Reviewed By: #bitcoin_abc, PiRK, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8173

doc/unit-tests.md

@@ -62,9 +62,10 @@ which you want to create unit tests. The file naming convention is
 a test suite called `<source_filename>_tests`. For an example of this pattern,
 see `uint256_tests.cpp`.
 
-For further reading, I found the following website to be helpful in
+For further reading, I found the following websites to be helpful in
 explaining how the boost unit test framework works:
 [https://legalizeadulthood.wordpress.com/2009/07/04/c-unit-tests-with-boost-test-part-1/](https://legalizeadulthood.wordpress.com/2009/07/04/c-unit-tests-with-boost-test-part-1/)
+[http://www.alittlemadness.com/2009/03/31/c-unit-testing-with-boosttest/](http://archive.is/dRBGf)
 
 ### Logging and debugging in unit tests