RFR: 8273497: Building.md should link to testing md file rather than html

[Dan Heidinga]

On Wed, 8 Sep 2021 14:20:04 GMT, Dan Heidinga <github.com+8503711+DanHeidinga at openjdk.org> wrote:

> Discovered while working through the building guide.

Replaced by https://github.com/openjdk/jdk/pull/5451

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PR: https://git.openjdk.java.net/jdk/pull/5417