RFR: 8274083: Update testing docs to mention tiered testing [v6]

[Aleksey Shipilev]

> Now that OpenJDK has more or less complete `tier{1,2,3,4}` definitions, let's mention them in `testing.md`. 
> 
> Current patch is my braindump, I am open for suggestions :)

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Another minor touchup

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5615/files
  - new: https://git.openjdk.java.net/jdk/pull/5615/files/efd28804..94496373

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5615&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5615&range=04-05

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5615.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5615/head:pull/5615

PR: https://git.openjdk.java.net/jdk/pull/5615