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

[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.

-------------

Commit messages:
 - JDK-8296240: Augment discussion of test tiers in doc/testing.md

Changes: https://git.openjdk.org/jdk/pull/16384/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16384&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296240
  Stats: 11 lines in 1 file changed: 9 ins; 0 del; 2 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