[jdk11u-dev] RFR: 8274083: Update testing docs to mention tiered testing

[Aleksey Shipilev]

Clean backport to improve documentation after introducing consistent `tier{2,3,4}` in related backports.

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Commit messages:
 - Backport 971aa353aae51222ca33ebfdd61b91672a6c871a

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/478/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=478&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274083
  Stats: 60 lines in 2 files changed: 60 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/478.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/478/head:pull/478

PR: https://git.openjdk.java.net/jdk11u-dev/pull/478