RFR: JDK-8296240: Augment discussion of test tiers in doc/testing.md [v4]

[Joe Darcy]

> Clarify the intention of tier 1 tests. I'll reflow the paragraph and regenerate the HTML file once the wording is agreed upon.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Implement review feedback.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16384/files
  - new: https://git.openjdk.org/jdk/pull/16384/files/6a0dac7b..688e9b67

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16384&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16384&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16384.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16384/head:pull/16384

PR: https://git.openjdk.org/jdk/pull/16384