RFR: Correct the build/test link to use the actual official links

[Iris Clark]

On Thu, 9 Sep 2021 11:31:41 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> It was recently pointed out (in the discussion of JDK-8273497) that the Guide does not use the official link to build instructions, despite claiming so. Correct this. Also use official link for testing.

Marked as reviewed by iris (Reviewer).

-------------

PR: https://git.openjdk.java.net/guide/pull/64