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

[Dan Heidinga]

Discovered while working through the building guide.

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Commit messages:
 - 8273497: Change from testing.html to testing.md

Changes: https://git.openjdk.java.net/jdk/pull/5417/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5417&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273497
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5417.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5417/head:pull/5417

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