RFR: More on testing

[Jesper Wilhelmsson]

* Added clarifications and expectations around testing and review
* Moved GHA section (unchanged)
* Removed outdated stuff from the glossary

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Commit messages:
 - More on testing

Changes: https://git.openjdk.org/guide/pull/98/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=98&range=00
  Stats: 43 lines in 4 files changed: 21 ins; 19 del; 3 mod
  Patch: https://git.openjdk.org/guide/pull/98.diff
  Fetch: git fetch https://git.openjdk.org/guide pull/98/head:pull/98

PR: https://git.openjdk.org/guide/pull/98