RFR: 8295663 Rephrased introduction to testing

[Ludvig Janiuk]

Old one made little sense for a new reader

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Commit messages:
 - ws
 - Rephrased introduction to testing

Changes: https://git.openjdk.org/jdk/pull/10763/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10763&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295663
  Stats: 38 lines in 2 files changed: 20 ins; 3 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/10763.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10763/head:pull/10763

PR: https://git.openjdk.org/jdk/pull/10763