[jdk17u-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 [JDK-8317228](https://bugs.openjdk.org/browse/JDK-8317228) in 17u, 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/jdk17u-dev/pull/2236/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2236&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323717
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2236.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2236/head:pull/2236

PR: https://git.openjdk.org/jdk17u-dev/pull/2236