[jdk11u-dev] RFR: 8323717: Introduce test keyword for tests that need external dependencies

[Aleksey Shipilev]

Semi-clean backport to allow running all tests cleanly. There was a contextual conflict in `TEST.ROOT` due to the absence of many keyword clean up patches, which I think can be backported and resolved later.

Additional testing:
 - [x] `applications/jcstress applications/scimark` are now properly skipped with `JTREG_KEYWORDS=!external-dep`

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Commit messages:
 - Backport 12b89cd2eeb5c2c43a2ce425c96fc4f718e30514

Changes: https://git.openjdk.org/jdk11u-dev/pull/2569/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2569&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323717
  Stats: 62 lines in 32 files changed: 32 ins; 0 del; 30 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2569.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2569/head:pull/2569

PR: https://git.openjdk.org/jdk11u-dev/pull/2569