RFR: Initial section on testing

[Jesper Wilhelmsson]

It's a start.

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Commit messages:
 - Initial section on testing

Changes: https://git.openjdk.java.net/guide/pull/30/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=30&range=00
  Stats: 127 lines in 2 files changed: 109 ins; 16 del; 2 mod
  Patch: https://git.openjdk.java.net/guide/pull/30.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/30/head:pull/30

PR: https://git.openjdk.java.net/guide/pull/30