RFR: 8342836: Automatically determine that a test in the docs test root is requested

[Nizar Benalla]

Please review this patch to automatically set `TEST_DEPS` to `docs-jdk` when testing the documentation.

This comes as a followup to https://github.com/openjdk/jdk/commit/07f550b85a3910edd28d8761e2adfb8d6a1352f6 , as review comments suggested better aways of adding dependencies and to not require users to use a verbose `make test TEST=docs_all TEST_DEPS=docs-jdk`.

I have verified that `make test-docs_all` does indeed build the docs, `make test TEST=docs_all` does not but the former is clear and short enough to be used.

TIA.

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Commit messages:
 - set an alias for testing docs

Changes: https://git.openjdk.org/jdk/pull/22239/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22239&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342836
  Stats: 12 lines in 2 files changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22239.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22239/head:pull/22239

PR: https://git.openjdk.org/jdk/pull/22239