RFR: 8274083: Update testing docs to mention tiered testing

[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 :)

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/5615/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5615&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274083
  Stats: 41 lines in 2 files changed: 41 ins; 0 del; 0 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